You can't. Maybe use a curtainwall with Edit Profile for the railing instead...?
Create custom panels for those...
Have you tried starting out with a flat shape & then Modify Sub-Elements? You can add pick points at very small intervals, raising the profile gradually to the mid-point, then back down. As a series of short straight lines - it might accept the railing to follow. I would probably try the railing as incremental segments, too - I cant see a top rail making an arch.
I just tried my proposed method and, you're right, it doesn't work. So? In a pinch make an in-place model...
After trying out my earlier suggestion and a few stabs at different iterations, I came to the conclusion that railings are hard-coded to be strictly linear. BUT WAIT! I went back to a current project where we have a circular staircase - and the railing is sooooo nicely and appropriately curved. So - WTF?
Railings have always been largely frustrating (even with Phil Read's cute tricks) and usually a big wish list item.
Maybe the Factory chaps are hard at work with some Elves brewing up a brand new Holiday present.