I’ve used a large, heavy fondant plaque on a buttercream cake (on the Archie cake I made last month) and the sugar glue helped, but I did put 3 toothpicks underneath to support it. As for draping, I’m not sure how you’d be able to secure it since they can be quite heavy. Might be worth a test run with fondant or modeling chocolate … try some sugar glue, piping gel or chocolate to attach?
Hopefully someone else has tried this and can advise you. Sorry I can’t be of more help. :)