> if I set my mask as /27 (or, in
> old style, 255.255.255.224), then the .144 host is no longer the zeroth
> one in my subnet.  But then, I lose connectivity to the .128 through
> .143 hosts, since they aren't on my LAN even though they should be based
> on the mask.

Just add a static route for that other /28 down the gateway.  NetBSD
does CIDR, so you're allowed to have routes to enclosed subnets
pointing elsewhere.  And configure the rest of the hosts on your
network with the correct /28 netmask, default routing to the netbsd
box.  (assuming that they too will refuse to use host 0 as the
gateway and unlike netbsd, you might not have source code to them :-)

okay, so you have an extra hop and you'll have to sysctl to stop
your netbsd box sending redirects, but hey, it'll work.

Kludge or what ...  Yeah, I've built uglier networks and the
network architecture police haven't arrested me yet :-)

> Is there any hope for me?  Is there anything I can do to make ARP handle
> the zeroth host?  Or am I doomed?  :)

One could always UTSL, but with an adequate workaround, I'm not sure
the motivation is there :-)