Hmmm. OK.
I haven't really got enough into groovy to know what it can do network wise... but by the time I need to deploy another server or similar, we're getting outside the limits of what's convenient to do to my mind. Equally, a 15-minute latency wouldn't kill me for presence detection, but then it would be a question of knowing that it had been seen in that timescale.
Is SNMP an option, perhaps? Query the border router's ARP table, and actually, that's not a bad proxy either, depending on the aging of the records. I suppose not everybody has a border router that'd play ball.
