Much in the same way that parallel dipoles can be fed with a common feeder,
can 5 delta loops be fed via coax matching stubs with a common 50R feeder?
(i.e. bring the ends of the matching stubs together?).

Not necessarily no, but I think it will give you a headache. If you
don't have antenna modeling software on your side, you're going to have
to do it experimentally and you're probably not going to get much help
on the internet. You might consider going with full-wave rectangles
that are twice as high as they are wide. They don't need any matching
stubs; they're 50 ohms to start with.

There will be big interaction among any 5-HF-band loops at any rate.

You might do better with switching all five feedlines with a remote
coax switch of some description.

73,
Dan