I now succeeded by issuing

echo 1 > /proc/sys/net/ipv4/ip_no_pmtu_disc

on my tftp server machine (which happens to run Linux 2.4), as  
mentioned on http://www.gentoo.org/doc/en/handbook/handbook-mips.xml? 
part=1&chap=2
if anyone could explain why this is necessary I'd appreciate it.
Anyway, I could successfully boot netbsd (only to have the kernel  
tell me that it only supports the serial console) and Gentoo Linux  
(which wants to boot of from an NFS server which I haven't set up yet).
Looks like NetBSD doesn't support the O2 framebuffer device yet?

-- 
Georg Schwarz    http://home.pages.de/~schwarz/
georg.schwarz@freenet.de  +49 151 11559652