So according to Github, the issue has finally been fixed.
Would anyone be willing to compile a Yosemite-compatible build?
Sure, just replace the main Stepmania.app file with this (decompress zip first of course):
https://mega.co.nz/#!NxhXhCLK!xh87yDLVYgy70Uy5pPFO3pOHQo4wVC4C0tBOUiNYkSY