If one were to boot NetBSD from a CD-ROM, make no use of local hard disks,
   and wished to provide some temporary writeable storage, would mounting an
   MFS on top of the ISO-9660 FS with a union mount work (i.e. is union stable
   these days, and does that combination of FS's work)?

why do you want an unionfs?  just mount a mfs somewhere and use it..
with symlinks, etc..
   
   The other thing that such a configuration would have to deal with is not
   having any swap space configured - is this something that UVM can deal with
   gracefully these days?

UVM deals with real memory shortages far better than mach VM did, mostly
due to the work done by chuck silvers as part of the general paging work
he did.  but both have always capable of operating without swap, at least
in netbsd.  infact, NetBSD/sparc originally was _unable_ to swap.  i used
to kill everything else except my kernel compile on my 16M SS1+, just to
make sure i didn't run out of ram and then hang..