Alright I was looking in all the right place... just not hard enough. 
there's a connect/disconnect event available directly from hostapd.
opkg install hostapd-utils will give you hostapd_cli
which you can run with an 'action file' (e.g. a simple shell script)
hostapd_cli -a/bin/hostapd_eventscript -B
the script will be get interface cmd mac as parameters e.g.
#!/bin/sh
logger -t $0 "hostapd event received $1 $2 $3"
will result in something like this in the logs
hostapd event received wlan1 AP-STA-CONNECTED xx:xx:xx:xx:xx:xx
hopefully somebody will find this helpful someday