I would be very interested to know how this turns out, because I made a remix of Fort Minor's "Remember the Name."
However, the samples and acapellas I used were released to the public by Mike Shinoda (the artist), himself, for the very purpose of allowing people to make remixes of his song.
If I am requested to remove it from my account, I will not hesitate to follow the directive. However, I would appreciate it if that could not be the case, as I put a great deal of time and effort into creating the track in the first place. I honestly think that it should be alright to use accapellas if the accapella has been released to the the public by the author and/or producer. Any other situations should not be allowed.
Perhaps we should have it inserted into the tos that the user certifies that they have the right to use the content, otherwise they accept that their account may be deleted.