per dhcrelay man page:
"dhcrelay supports relaying of DHCP traffic to configure IPsec tunnel mode clients when listening on the enc(4) interface. The DHCP server has to support RFC 3046 to echo back the relay agent information to allow state-less DHCP reply to IPsec tunnel mapping."
also
command line switch -o = "Add the relay agent information option. By default, this is only enabled for the enc(4) interface."
I believe OpenBSD 4.7 dhcpd supports RFC 3046.
If I'm correct, then it's just a matter of making it work...