It looks from my reading from of dhcpd.c that you could just tweek windows (the registry I assume) and make windows ask for the domain, in which case dhcpd should supply it.
If you hate this idea then I think the change to add windows specific dhcp options would be easy - there is already a special case for plan9 clients. FWIW there is another weirdness of windows, the windows DHCP server doesn't communicate with the DNS server on windows, it expects the client to send an Inform packet to the DHCP server telling it of the clients chosen name. -Steve
