To explain why bother:

As it stands, oldwikisource is almost invisible.  It can't be linked via interwiki and it is not, as far as I can tell, supported by Wikidata (this was partly prompted by trying to get its author pages linked up properly).  Lots of people don't even realise it's there.  Oldwikisource gets almost no traffic or attention.

"Mul" is the correct ISO-639 code for "Multiple languages", just like "en" means "English", "fr" means "French" and so on.  Several people already refer to the project with the shorthard "mul.ws".  (For the record, the other official no-specific-language language-codes are "mis" for uncoded languages and "und" for undetermined).

I know there is a bugziilla request (52971) to catch interwiki links to unused subdomains and redirect them to either the incubator, betawikiversity or oldwikisource.  That might solve some of this, but not the following:

There have been some objections to hosting a bilingual book on English Wikisource and the same may hold elsewhere.  Mul.ws could operate as more than just an incubator; it could be the primary project for works like this.  It would also leave the subdomain-less page as a language portal, like most of the other projects, and let mul.ws have a proper main page (presumably linking to all of its other main pages but still a viable page in its own right).

However, I don't know if this is really a good idea, if it would have any support, or if it would be too difficult to be worth doing.