(resolved, follow-up)

----- Original Message -----
From: "Manuel Bouyer" <bouyer@antioche.lip6.fr>
[...]
> Hum, reading lib/netif/d80x3.c, I suspect you may want to add:
> -DBASEREG=0x280 -DBASEMEM=0xd0000
> to CPPFLAGS. By default it's at 0x240/0xd0000

This worked; thanks!

This got netboot.com to find the card and successfully send a bootp request.
I think my problems now lie with bootp.  I'll start another thread.