MFS is simply a filesystem, which is convenient for setting up a /tmp, or 
   something.  To get the same on a ramdisk, you'd have to newfs the 
   ramdisk, which is a hassle.

True, but there's no particular reason that mount_mfs can't be changed
to:

1) Make a ramdisk image.
2) Make a file system on it (preferrably by actually calling out to
newfs(8)).
3) Mount the ramdisk file system.

This would make somewhat more sense than duplicating the functionality.