Serving Creme Brulee to a large group of people
To be honest I wouldn't have thought the top would get soggy. I've also made hundreds of brûlées for wedding parties in the past and the top has never gone soft. Unless you're putting them in the fridge between colorizing and serving? That will definitely cause problems.

If you are actually getting a soggy top it means one of two things:


You've not browned them enough.
You've not used enough sugar on the top.


One thing I could suggest if you are doing so many in one go, something I learnt quite late into being a pastry chef (6 months of a two year stint): make a nice perfectly coloured caramel, leave it to cool a little, but keep warm. Then when you need it just pour it over the top of your brûlées as you're serving. 

Be sure to use glucose in your caramel to stop it setting too hard, as it may splinter and kill a couple of your guests.