As a first glance, it seemed like IMSLP was only for scores in the public domain, but then I found this:
So it seems the work creator can also publish scores to the site under, for example, Creative Commons licenses. This I what I would suggest to do. To quote Cory Doctorow (who also happens to be a MetaBrainz board member!):
(From the preamble to his novel "Little Brother" (available for free on his website).)
But eh. It's your decision to make, and people who work on open source, open data projects are unlikely to be completely unbiased on this matter.
If you do end up posting the scores to IMSLP, note that you can link the IMSLP pages to the Work entities in MusicBrainz. (And the same for lyrics, if you post your lyrics to Lyrics Wikia or some of the other white-listed lyrics sites.)