Hello @dan_kreider to carry on from our PM’s bringing a few points here. The sites for services I run have a PayPal Donate button for support for running costs. You could say this is ‘pay what you like’. I just call it donation. An issue that comes up from experience is that there is a flurry of financial support at the beginning and after a year nobody donates any more, unless you mail all the members and prod them. A perennial problem.
From a couple of points from the original post that started this off, I don’t think this concept is of much interest to professional composers who generally don’t share for IP reasons, but hugely valuable for educational and example purposes.
As to copyright, the way I would deal with this is to simply state the policy in a user Guidelines and Policy page, and enforce it via admin action if violations are found. I don’t think you have to get involved in learning how to be an international copyright lawyer, as it’s pretty clear when something is actually copyright material. This implies the site should support user membership, and an admin interface, not just wide open public access with no accountability.
Speaking with my software architect/developer hat on I don’t know at present of any open source platform that does this exact sort of thing out of the box. I feel that the Musescore library is a very good interface, and so is IMSLP. But I am convinced these are both bespoke software developments.
Regarding scores that are available elsewhere such as IMSLP, I think the intention is to make a repository of Dorico files for use and study, and these are not available grouped for fluent access anywhere at present.
Now here’s one use case that makes a project like this hugely valuable just in this one case: an indexed browsable repository of Expression Maps, instead of having then buried in distinct posts on the Dorico forum, not easily retrieved. Personally I feel that would make it really worthwhile.
I strongly believe that if the site is on a well designed platform and has an attractive and fluent UI that people will use it. I have given this serious thought over the last few days, and sent @dan_kreider a sketch of what I see as user requirements and software platform requirements, and even some estimate of costs.
Most definitely such a site needs membership, moderation, and admin roles, if for no other reason than to avoid being spammed with endless stuff, ads, etc.
I initially proposed in the original thread that I would be prepared to develop this., and that still stands. You could almost bend Owncloud to this purpose, it supports large databases, good interface for uploads and browsing, tags - very important - and search by tag, but it’s not quite exactly right. I self-host OwnCloud and it works great, but something the next level is needed. I am continuing to research what the best platform architecture would be.
In short, a good idea, but it needs to be done well to be a success. To that extent I don’t believe just trying to use Dropbox or other cloud file storage would be a hit with users.
Finally, I’d like to see it curated to assure a certain level of quality - that works are finished and complete for one - but I know that idea will go down like a lead balloon. 
