The release notes for R83 indicate that the new wifi/wired presence detect functionality only works in router mode. I'm guessing this is based on DHCP leases in order to allow it to work for wired clients as well. Any chance this could be extended to work in AP, e.g. by using 802.11 association status (i.e., what's shown in the "associated stations" list on the OpenWRT status page) instead?