It looks like that may well be what happened - it seems MFS will, over
   time, grow to use up it's full size in swap space, even with activity
   on just small files such that it really should never need to touch
   more than a very few of it's pages.

That's because MFS uses the same allocation strategy as FFS, which
ends up cycling through most of the blocks in the file system
eventually.  It really needs a different block allocation algorithm.