The easiest way to achieve this would be to separate the clients into different SSIDs/WLANs. Place the clients who need addresses from the Built-In DHCP server in one, and the clients who need addresses from the External DHCP server into another.
This way, you could point the Proxy DHCP server IP address wherever you need it to go depeding on which client is requesting an address.
(In my current environment at $DAYJOB, we're utilizing a similar configuration. We're actually sending the proxy DHCP request to 3 different locations depending on the SSID.)
You could change the destination for the DHCP requests either on the Interface corresponding to the WLAN or on the Advanced tab of the WLAN itself.