The AP is totally unaware of any IPs involved since packets are bridged from the wireless to the tunnel interface/LAN interface. The ASG doesn't even see those packets at all (if you use bridge to LAN, for example).
Aside from actually using packet sniffing to listen for DHCP traffic, we didn't come up with a "universal" solution to this problem. Currently we're considering this as being too hackish and lacking elegance (even we have standards [:P] ). But we know it would be much better to have a solution that just works, no matter what setup is involved. Part of the beta is always to see what deployment scenarios are out there and to see what is missing, so your requests are being noted...