I can't claim to be a lawyer, but I feel that if Hasbro didn't like it, that they could still cause trouble. I don't ever expect companies (especially their legal departments) to act like adults or even play fairly. Theoretically I believe that Hasbro could attempt to sue pony.fm for donations. That is all they would need because they could easily stall and win that way because there is no way we could hold up financially against Hasbro.
That being said, I've seen a lot of songs that are their completely their own song with the only connection to the show being it's inspiration. Silva Hound, for example, sells "MLP" music that are not remixs of anything, but are instead inspired by certain characters in the show and the fandom. Sim Gretina also does this and I am sure there are more. I've already seen a lot of these kinds of songs on pony.fm and it would be neat if they could have the option to sell them.
If the money ever becomes available it might be worth it to get some legal consultation on the matter. Perhaps something can be worked out that pushes the responsibility for the content onto the user similar to the way youtube protects itself from users posting copyrighted material.
At the very least you might want to consider adding soundCloud and Bootcamp links to the profile/individual song/album pages.