Ok, I have tested whether it works when externally linking using the new Limited Reference option, and unfortunately, it does not (yet). I can't see a reason why it shouldn't, though. So this would be an easy ER: the main scene should not claim externally linked geometry that is linked via Limited Reference. This would instantly enable the described kind of collaboration. In addition, there should be a command to convert traditional links to a limited reference links, so older files can be updated easily.