Everyone who suggested using bootp or dhcp was correct.  
Pmax PROMs don't use RARP to get their IP address, they use
bootp. Therefore you have to set up a bootp server to boot a
DECstation diskless.

Rather than getting the diskless-boot server and path from bootparams,
the NetBSD kernel gets it from bootp/dhcp, too.  That saves you from
setting up a server.  (I was sure this was mentioned in the INSTALL).

Another point worth noting is that NetBSD-current dropped bootp server
support, so you might as well set up a dhcp server now. It's easy.
Try it, you'll like it.

Here's an example (lightly edited) based on I use at home:

------- begin dhcpd.conf
# we are an unoficial server.  Don't answer to unknown clients.
deny unknown-clients;

subnet 10.103.0.0  netmask 255.255.0.0  {
    option subnet-mask 255.255.0.0 ;
    option broadcast-address 10.103.255.255 ;
    default-lease-time 600 ;
    max-lease-time 7200 ;
    option routers 10.103.0.1 ;
    option domain-name-servers 10.103.0.36 ;
    option domain-name "Stanford.EDU" ;

# tftp pathname is "netbsd.ecoff"
# diskless root is at "/export/nfsclient/pmax-root"
host cuisinart {
     hardware  ethernet 08:00:2b:2e:7c:ed ;

     allow bootp;
     allow booting;
     fixed-address 10.103.0.50 ;
     filename "netbsd.ecoff"  ;
     # Warning!
     #  pmax proms  (e.g., KN02-CA v2.0m PROM) use bootp, not dhcp,
     *  and there is a hard limit on option size.  The PROMs fail to get
     #  an IP address if option  root-path has a string of >= 16 chars.
     # Use symlinks to keep the root-path <= 15 chars.
     # OK:        option root-path "xxxxyyyyzzzzaaa";
     # TOO LONG:  option root-path "xxxxyyyyzzzzaaaf";
     # ALSO TOO LONG: option root-path "/export/nfsclient/pmax-root" ;
     # OK ALTERNATIVE:  
     #   (after doing "ln -s /export/nfsclient/pmax-root /pmaxroot"
     #   on the NFS server)
     option root-path "/pmaxroot" ;
}
------- end dhcpd.conf