Modifying DHCP relay or forwarded responses

I'm using openwrt devices as vpn nodes to extend our network to multiple sites. What I'd really like to do is relay dhcp requests back to a central dhcp server. Then add/replace the default route, and possibly make other modifications to the response before returning it to the client.

If dnsmasq, or dhcrelay have a functionality such as this, I don't see it.