Norbert,
I am replying to this old post, as I have much the same question and a possible way forward.
I use an OSpot 4+ with a D-Star radio and I am able switch connectors very effectively by having a memory set with URCalls such as XRF678BL . The difficulty is that the OS4+ interprets anything set in the Radio's URCall field as an instruction for the local OS4+. This makes sending a routing command or even VIS_____ON to a distant server impossible.
A possible solution might be to allow an Escape character such as & which does not occur in any call sign or OpenSpot command. &AB9CD could then be forwarded on to the server as you suggested in your comment "server can handle direct calls or other command".
At the moment this can't easily happen as the OpenSpots try to handle any further instructions locally.
Regards
John