How well does UVM deal with not having any swap space?

fairly well -- paging was one of the last things added to UVM.  it
could be better...but it will be fine.

   How hard would it be to properly support such configurations (i.e. no
   hangs, crashes, etc, if RAM exhaustion hits)?

that really isn't a property of enabling swap or not -- and thus
UVM will lose equally badly in both situations.  netbsd does not
currently do page preallocation for things like COW areas, which
can still cause deadlocks... (swap or no swap)