After a great battle, I got my Sun-3/280 booting again! HURRAH! The second ethernet interface isn't working, which is greatly annoying, and many other things are limping, but I can at least telnet in.
Net booting has to take place on a little private ("vintage") subnet to avoid bootparams conflicts with dhcp. Apparently Freebsd bootparamd does not work with Sun-3s, so I have to have a raspberry pi whose only task is to direct the sun towards the correct freebsd nfs mounts.
Will fix later.
(This is a VERY similar environment to the VAX-11/750 running 4.3BSD that I first learned Unix on, so it's very nostalgic for me.)