1. Is unionfs stable enough for this?

well, probably.  it seems to work OK if you don't access it at multiple
concurrent times (eg, 'make -j6 build' :).
   
   2. How does one mount an MFS (which has no node in /dev) over / with unionfs?

here is my MFS fstab line for my /tmp:

	swap	/tmp	mfs	rw
   
   As for the swap question, it really devolves down to the resource
   accounting issue, which you get into whether you have swap or not: how well
   does NetBSD deal when there is no more RAM (or RAM+swap)?
   
   I know that the MACH VM had this notion of "lazy allocation" which allowed
   for large, sparse address spaces, but lost terribly if a resource crunch
   hit. I also know that we've been tightening up on the resource accounting
   with UVM, but it's not clear to me what we do for the "sorry, we're all
   out" case right now.


UVM is also lazy allocation.  when UVM detects a out of ram completely
condition and a lazy allocated bit of memory is accessed, the offending
process will be KILLed and the system will continue.  there is also a
small buffer of reserve pages only for use by the paging system, which
allows the pagedaemon to run comfortably when there is otherwise no
free ram.  most of this work was done by chuck silvers.