Chances are slim to none, since firefly relies on the DAAP (iTunes) protocol, which needs to be licensed.
As for the RSP client part, I am quite certain that RoKu would pursue similiar goals, or at least put their code under LGPL or BSD-Style-License….. 🙁
Anyhow, as Firefly is plugin-extendebla, it surely would be a sweet project.
AD Blackfin eval boards are rather cheap, and giving lots of power. They only lack a display, but thats not the hard part to glue some logic and a screen to these things.