I ended up putting in 8'x10' of 3/4" (20mm) high density foam, I have another 20-30sq ft I can put in if I decide to run it wall-to-wall-to-wall. It's not as strong as rubber would be, but it's nice under foot, and I can top it with a carpet for a little more resilience.
I may be putting in some Dance-y stuff too
I don't know the first thing about Barre excercise, but my lady needs to do quite a bit of rehab on her shoulder/arm, and a set of Swedish bars/ladder might be a good thing for it.