Andreas, that's exactly as I use the button on my Royal Pro, to enact a flight phase for the discus launch on a DLG, when you throw the glider you hardly have time to get on the sticks, releasing the button then reverts to a normal flight mode at the apogee.
The C2 Sx had the fixed function which was very useful for this too.
I don't have the latest Cockpit Sx but do program my friends one, try reassigning the vario/teacher function to a different switch and maybe this will then free the labelled button for other duties, I don't know if this will work but give it a try.