Finally I could sort this out:
the llInstantMessage command works as it should, if triggered from a touch-event or from a timer-event
- however if triggered from a sensor-event, it will only work within the same region and for the distance set in the range-parameter of llSensorRepeat.
Not sure if this is intentionally like that in OpenSim (not sure about behavior SL)
Thus to solve the visitor registration & IM-message X-region to owner/staff I scripted a workaround that works nicely now and is in beta-testing.
I'll make it available in Kitely Market and @ my sandbox soon, when it has been testet more thoroughly.