At least in R:TW unlike M:TW, if a human faction bribed a demon faction then the units would just disband rather than join the humans. Even still though, what would demons do with money? So if possible diplomatic relations with demons should not be possible.

Now here's an idea. Could the human factions be able to acquire 'turned-demons.' I mean, its not like all the demons have to be evil - some demons might be capable of some degree of self control. They could be like elephants in terms of AI, in that, they will fight for the human faction because they want to, but if it all gets too much they just go on a general rampage - because deep down inside they are still demons.