The two ways that I currently believe work are:

	(1) dd the miniroot onto the beginning of the disk, and then
	    use the Ultrix disklabel command to create a label with the
	    a partition starting at zero and at least enough bytes long
	    to hold the miniroot.
or,
	(2) disklabel the disk with partition a starting at an offset of
	    sixteen from the beginning of the disk, and then dd the
	    miniroot onto the disk with skip=16.

Both of these have been reported to work fairly recently, although I
have to admit that I haven't ever booted NetBSD/pmax this way.

			       _MelloN_