The implementation that I've seen of this is done by fiddling with the MAC forwarding table on the access point. Since the access point simply acts as a network bridge, it is fairly well suited to this kind of task. At the switching layer it is already collecting all of the heard (sometimes called learned) MACs and which interface it can be found on.
The logic looks kind of like this:
- Access Point receives a packet over the wireless interface
- Bridging subsystem examines packet for destination MAC
- If destination MAC is in the learned switching table for wireless interface -> DROP
- Otherwise forward packet via wired interface
Because of the way network bridges work I see this being fairly difficult to trick the access point into forwarding a packet to a client in spite of the isolation. Your best bet would be to attempt to talk directly to the other client, as if you were operating with an ad-hoc network.