Depends on what you try to do.
Starting a “UI” program from nsclient++ should work I guess, but it wont start on the “users screen” as the service runs in another session (0) and users run in >=1.
So it depends on what you try to achive, I would say it is probably nott a good way to do it, external scripts are designed to be small and fast laucing UI:s seems not the right way to go.
Cant you “talk to the program” instead?