Waiting for external input on a waypoint (Copter/Rover)

Hi all,
it must be simple (at least I hope) but I cannot find the required information. Hence, I need your help!
My aim is: when a rover/copter reaches a waypoint it should
1: stop
2: send a trigger signal (e.g. like CAM_TRIGG_DIST in APM:Copter)
3: wait for a return trigger signal before proceeding to the next waypoint
It would be even better if CAM_TRIGG_DIST or something similar could be used instead of a waypoint because it would make mission planning much easier. In this case the rover/copter should stop when CAM_TRIGG_DIST is executed and wait for some input before proceeding further to the next waypoint (or the next CAM_TRIGG_DIST location).
The general idea is to stop the rover/copter until some external processing is finished. So, a simple delay is not an option.
Is something like this possible?
Thanks a lot in advance,

Too easy or not so easy?