An update on the HP/NetBSD front: and that's the only thing that's up.
The boot process requires NFS. I don't trust NFS (never have, never bothered to install it on the home system here). Sigh. Download, install, configure.
So I have rbootd and rarp working on Linus. Turn on the HP. Using some network monitoring software I wrote (a near-clone of tcpdump that outputs in a more concise mannor) I can see the HP making requests of rbootp.
And one lone NFS packet:
IPv4 208.26.72.7 255.255.255.255 UDP 1023 111
So far no luck getting past that.