> The U.S. has the concept of fair use see 17 U.S.C. § 107
But we want LilyPond to be distributable in more than just the U.S.A.
Indeed. I am not a legal expert by any stretch (I've just read a lot of stuff on copyright law, between this project and some other related interests of mine). In particular, I would say that anything used should be incontrovertibly in the public domain (i.e., the older, the better).
1) While many jurisdictions recognize the rule of shortest term, this is not a guarantee, particularly if there is a specific agreement between two countries. For instance, I think the U.S. and Germany have a bilateral agreement that says each handles copyright according to its own laws, regardless of the country of origin.
2) As David has implied, "Fair Use" varies widely from jurisdiction to jurisdiction, if it exists at all in a jurisdiction.