I tried the same thing a few months ago.
If you have the guest SSID set up to deny access to local intranet then that also disables the guest network from accessing your DHCP server. What I ended up doing was keeping the DNS server, etc. running on the Windows Server, but using the router as the DHCP server. <-- you must disable the DHCP server process on the Windows Server or you will run into problems down the road, Windows Server disables DHCP if another DHCP server is found, but if something happens it could start back up and start confusing devices.
I do not see any settings for RIP, etc. allowing the router & server to share DHCP responsibilities. (Running Merlin 270.24).
If there is a script or anything that would enable that functionality I could not tell you as I have not dealt with scripts or SSH or termial or anything of the nature with the ASUS router.
Since you are running a domain, most of your local network resources should be based on access control correct? Thus you really should not have a problem allowing intranet access from guest network.
Last edited by wvaj; 02-21-2013 at 10:11 PM.
Reason: Added info after arrow