I was actually with my brother at a restaurant that had karaoke couple months ago, and it occurred to me that the idea of 'music-minus-one' for instrumentalists would be very, very useful. As a violinist, I know I've always wanted to 'play along' with the piano accompaniments for many of the great sonatas. Many people do not have the opportunity to play with real accompanists, and it's something that can be both fun and useful for concert preparation. Especially for concertos. I've searched all over the internet, and can really only find specialized files for this, but they are very few 'karaoke' files for classical music. I would say there aren't more than 100 in all.
I think it would be a really useful addition to IMSLP, and I really do think a lot of users would find this addition to be really beneficial. I was thinking that it would certainly justify a new tab in the recordings section (next to synthesized / MIDI).
Sadly, there are not very many 'MIDI minus one' files already existing, but I will try to seek permission to post a few if people here generally agree that this might be a good idea.
It also does not have to be limited to just MIDI files - real people can play these parts as well of course. Initially, it might be easy to take some FINALE or SIBELIUS renderings and extract one of the parts.
In theory, I do not see why we couldn't also do this for trios, quartets, etc, except that it might make the page a bit bulky.