Just out of interest, how would this help? An interwiki link for OldWS already exists (oldwikisource:). I believe the problem is rather the same incubatorwiki and betawikiversity have, that you cannot link an item multiple times to the same wiki. Anyway, there are already multiple wikis awaiting a rename but the WMF seems to be reluctant to fulfil these several years old requests. So even if a move to mul.* is decided by the community, it is probably not going to happen within the next 20 years.