Atg bigbrd's suggestion, I tried loading the ne.o module with the "bad=0xbad" parameter (which I hadn't bothered doing, since I assumed the bogus signature meant the ISA I/O was somehow screwed up).
The good new is that that works: both the board that had Doctor Vit's Miracle Cure Surgery and the one I set up on the DOS box without soldering are happy now. I even have one of them talking over coax (for those Manly Men who prefer The Original True Wire to that punk leftover phone company stuff 
).
The bad news is that putting:
ether=9,0x80400300,0,0xbad,eth0
or even
ether=9,0x80400300,0xbad,0xbad,eth0
on the boot command line didn't work. So I'll have to patch the driver to get around that for NFS-root booting.
Thanks to Vit and Bill for the help in getting this straightened out.
Ran
