I just created  ftp.netbsd.org:~ftp/pub/NetBSD/pmax/network-install/.

There's a copy of diskimage.tar.gz that can be unpacked and used as a
bare-bones NFS-mounted root, until you label a local disk and unpack
the NetBSD/pmax distribution (or snapshot) into the local disk.

here's a copy of the README.

--------
This directory contains installation material for installing NetBSD
1.2G on a diskful pmax over the network.  See the NetBSD/pmax
installation guide for more details.

Contents:
netbsd.ecoff.gz:	A gzipped, net-bootable copy of
			the  GENERIC netbsd/pmax kernel.
			If this boots via TFTP or MOP,
			use it.

nfsnetbsd.ecoff.gz:	A gzipped, net-bootable copy of
			the NFSINSTALL netbsd/pmax kernel.
			This has SCSI code and disk drivers,
			and should be bootable via TFTP
			even if netbsd.ecoff isn't.

diskimage.tar.gz:	A gzipped tarfile containing a copy
			of the contents of diskimage.gz.

			Untar this into the server directory
			which your pmax is going to NFS-moot
			as its root directory.
			Remember to specify -p when you untar.

Note: diskimage.tar.gz also has copies of the kernels, so that your
pmax can use them to build its kernel-symbol database with kvm_mkdb.