I'm actually currently testing the multi-file template. When IMSLP comes back online again, I see no reason why we cannot have both the scan and typeset
They won't take up much space (layout-wise) in any case.
About being "distributed", this may present itself to be the most problematic element. Normally, I would suggest no parallization unless absolutely necessary, for performance reasons. This is because, as anyone who has written multi-threading applications will surely relate to, negotiation between different people takes a rather large toll on the efficiency of the project if the parallization is not done correctly (which, according to Moore's law, is always the case haha). The best case scenario is if one person (and only one person) is in charge of an independent chunk (a piece, or even a movement).
This is of course slightly different with text, as text is considerably more straightforward. On the other hand, connecting different pages of music is not a trivial task (and to make it look good!).
To prevent stalled projects, we can have some sort of transfer function which will transfer incomplete typesets to another person who will work on them.
In any case, I would very much welcome it if someone can explore this idea more. I myself would be unable to lead such a project, but I, of course, welcome anyone who is interested to do so, and I will support it.