Our current GPS is only accurate to 7 metres, which might be bigger than your "stage (donated by the U S of A in 1983..... In 1983, the GPS ceased being solely a military system and was made available for public use. President Ronald Reagan declassified the GPS system in response to a tragedy involving a civilian airplane. Korean Airline flight 007 got lost over Soviet territory and was shot down by Soviet fighters.
Read more: http://www.maps-gps-info.com/gps-history.html#ixzz3WktAUrIQ )....".
The newer, European, non-military system is supposed to be accurate to 3 meters, but their satellites keep on "mis"-launching.
3 Metres is good for "turn left at the traffic circle" but probably not good enough for your purpose.
Gilberto's idea is probably the easiest to implement, with an overhead camera and movement recognition. It is already implemented in Video Trigger (by Zach Poff ), but only with 6 sensitive areas. If you run it twice (on 2 computers) with 6 horizontal bands for one, and 6 vertical bands for the other, with messaging between them ( you can use OSC and Midi if you run it as an add-in to H.C.Gilie's VPT7_01) then you would have 36 recognisable positions for your "actor".... enough? More cameras would give you greater definition.