There are many fictional accounts of this ball, starting with Thackeray's in Vanity Fair, and carrying on right up to Bernard Cornwell's Sharpe's Waterloo. Most of them (especially those that have made it onto film) have a happy disregard for details of what the ball would actually have been like. Many people have said that we cannot know exactly where it was held or how the evening would have progressed, which is odd as there are accounts of it by people who were there (including a convenient plan of the house where anyone who wants to look at it) and there are some modern accounts with details of the guest list and the food that was served.
I have not read all of the modern research, but my version is based on eyewitness accounts from the time and should give a reasonable idea of what the evening would have been like.
Here's a taste (edited to take out the spoilers):
The roads to the east of the Place St Michel were little more than lanes and the throng of carriages – and even the occasional coach – that the Richmonds’ ball had attracted meant that the drivers were reduced to jostling for position and they travelled well below walking pace. Indeed, had the road not been so dirty, Burke might well have suggested that they walk the last quarter-mile, but Lily’s dress was very fine and it seemed wiser to wait. The occupants of the other carriages had obviously come to the same conclusion and they sat, more or less impatiently, the ladies fluttering their fans and the gentleman staring ahead impassively as the lower orders gathered at the side of the road to gawp at this unwonted spectacle.It was not until eight that their carriage finally rolled up at the gate in the Rue de la Blanchisserie. A liveried gatekeeper took one look at Burke’s uniform and Lily’s splendid dress and barely glanced at the invitation before bowing them through.Other servants lined the carriage drive that swept to the rear of the house, directing them away from the main entrance that faced onto the road. Looking at the parade of vehicles ahead of them, Burke could see why they were being sent this way. There would be more space for the carriages to manoeuvre around the back. The Richmonds had a large garden and Burke expected much of it to be trampled into mud by the morning, but they could afford gardeners and such inconvenience was part of the cost of entertaining on a lavish scale.The arrangements for the carriages meant that guests entered the house through the doors at the rear, but these were every bit as grand as those at the front, normally allowing the Richmonds and their friends to walk as a group into the gardens. Now, with a canopy to protect guests in case of rain and a positive army of servants to hand ladies down from carriages and direct drivers, the entrance was as imposing as the Duchess might have wished.The doors opened into a hall that ran right through the house. Burke could see other servants waiting by the front doors, presumably ready for the guests of honour who would be speeded to that entrance without having to jostle with those being admitted from the garden. There was, he understood, a hierarchy of privilege even amongst those singled out as suitable to obtain tickets to the Duchess’s entertainment. Indeed, his own invitation was discreetly scrutinised in the hallway to see whether it was on the list of those who should be formally announced on their entrance to the ballroom or if he was even amongst those so specially privileged as to be guided first to one of the other rooms of the house, where they might be greeted in person by the Duke or Duchess. Being, though, a mere mortal, he was directed towards a passage off on the right, which led to an anteroom which, in turn, opened out into the ballroom.Given the excitement that gripped Brussels society over the evening, Burke was surprised that the room was by no means grand. It was long and low, and made to appear still lower by a gallery that ran around it. The gallery was crowded, as the floor was far too small to allow for more than a fraction of the guests to be dancing. Not that this seemed to be worrying anyone. There was a constant buzz of conversation, more animated than usual at such occasions. Besides the inevitable gossip about Lady John Campbell and the various other unsuitable companions that the Duke of Wellington had been seen with, Burke heard, unusually for such occasions, remarks about the deployment of troops or the likely plans of the French.They edged their way through the crowd and found space on the dance floor. To Lily’s delight, they were playing a cotillion. Burke suspected that she had their hostess to thank for that. The Duchess of Richmond was quite old-fashioned in her tastes and had probably insisted on some more traditional English dances. The cotillion was soon over, though, and it was back to quadrilles. Another cotillion and then, as if to make up for such unfashionable music, the band started a waltz. Burke, like every other man in the place, had practised the steps so that if he ever found a girl daring enough to dance it, he would not be found wanting. He was gratified to discover that Lily had obviously practised it as well. The two of them whirled around the room until the music stopped, their cheeks red with excitement, and half dizzy from turning so enthusiastically.The master of ceremonies took to the floor and asked that everybody should clear a space there. Burke and Lily found themselves pressed back towards the wall. At least they had the advantage of being close to a window, which allowed some much-needed air.Above them, the gallery creaked alarmingly as more guests gathered there to see the spectacle below, but any creaking was soon drowned out as the sound of bagpipes announced the entry of a body of Highlanders into the room.Burke’s own, brief, experience of serving alongside the Highlanders had not endeared them to him, but, unlike many Englishmen, he admired their music, and enjoyed the spectacle of the kilted figures as they marched onto the dance floor. The Duchess herself was in the first rank of the spectators and the enthusiasm betrayed on her face suggested that she was happy with the selection of the troops who made up this band. It was clear that they had been carefully chosen, for, even in a regiment noted for the size and strength of its soldiers, these men stood out. Marching in in their bearskins, they seemed as much like heroes of antiquity as soldiers in the British Army.As the cream of Brussels society looked on, the Highlanders performed a string of reels one after the other to the accompaniment of their traditional instruments. The reels were followed by a strathspey and then swords were laid on the floor so that the four handsomest of the company could demonstrate their skill in the sword dance. At the conclusion of the display, the men formed back into columns and followed their piper out of the room, to the applause of the crowd. It occurred to Burke that, if any of the rumours he had heard were true, they could well be marching directly from the ball to the battlefield.