I don't recall off the top of my head (it's been years since I wrote code in/with MPI), but I can't believe that it wouldn't be (easily) supported under NS. It should be easy to check online...
http://www-unix.mcs.anl.gov/mpi/mpich/OK, a quick check says no :(
As for PVM, I never liked it, so you're on your own there, heh.
On the other hand, NS/OS have Distributed Objects, which can be used for similar ends, but I don't have much info here.