Can anyone tell me if fondant swags will hold if draped onto cake iced in buttercream?

No. I told a co-worker not to do it. She did it anyway. The weight of it made the icing slide off. I wouldnt recommend it. Now....if you rolled it out paper thin....maybe it would work. In general I make it a rule not to do it though because of my experience with it not working all that great.

Thanks for the input, I thought that would probably be the answer.

I've done it and have to do it next Sat , but since it's for drapes I do make it THIN and do 50/50 fondant and gumpaste. The gumpaste is lighter and that helps. Did for me anyway.

Yeah the gumpaste probably helps you roll it out nice and thin too. I would say if your customer wants it and you feel comfortable trying it, then go for it. I did it once and wasnt happy, and then saw my co-worker try to salvage a disaster, so that is why I dont do it. BUT that doesnt mean its not possible. Are there any flowers or anything going on it that may help hide flaws? (like smooshed buttercream, or unintentional marks left while trying to position the drapes)