Summary: no, I don't think we should unlock this migration path.
As with all things unix, that to me is a bit of a special case.
I view it thusly:
Super User is for computer enthusiasts, who love OS X, Windows and Linux — and for that matter, all software and hardware — equally.
Unix/Linux is for people who love unixes of all flavors and types.. but probably hate Windows and only begrudgingly use Macs now that they are unix-y.
Ubuntu is for people who love just one type of unix: Ubuntu. They are on a holy mission to create the “unix for the rest of us”.
These audiences are really quite different in their approach, goals, and what they love and hate. I’m disinclined to allow a lot of arbitrary migration between them.
But in GENERAL, yes, we don’t want that kind of serious topic overlap between sites. Unix is just .. special.. in this regard. See Joel’s post: http://blog.stackoverflow.com/2010/09/fork-it/
[featured]meta question for migration target suggestions (with evidence) - unless
[featured]doesn't work here of course.