Frankly and respectfully, I disagree with your use of the word "hosting" in reference to the binary news group alt.binaries.sheet-music. It's just a news group. Newsgroups are having zero legal issues with distributing "I Am Legend" and "The Golden Compass" - two movies not even out publically on DVD yet. I'm not saying its legal, I'm just saying it is currently happening, and there are no legal repercussions for "hosting" these illegal files.
IMSLP is still being watched by UE, any small hint of using networks such as the one you are talking about (which utilizes illegal file sharing), they will jump at. We can not take a chance especially with such places.
Therefore, its hard to see a legal reason for not posting scores which we all agree are in the public domain in News groups.
Frankly again, it leaves me wonder what the real reason for not sharing is.
The law for copyright is very extensive, it contains many loopholes that UE could employ that we need to take into account. IMSLP is cataloguing every score, and either putting it in the US category, or the Canadian. This needs to be completed before any sharing of the archive will begin.
Therefore, its hard to see a legal reason for not posting scores which we all agree are in the public domain in News groups.
Posting in groups that openly share illegal works has a negative effect on IMSLP’s reputation, especially with UE. Yes, we may all agree they are in public domain in Canada, but they need to be sorted into the US server if US copyright, or the Canadian server if Canadian copyright.