Damn… @David-san Sir was quicker! 
I just nailed together a small scriptlet which might do the job as well… you can save it into the GP-Favorites folder and then insert it in the wiring view as a “Favorite”…
It should be placed right after the midi player. The transpose (+/- 12 halfsteps) can then be adjusted with the according parameter, which can also be mapped to a widget…
To save a particular channel from being transposed (i.e. CH10 = drums) i added another parameter for that purpose (thanks @David-san for the hint)
New Version with “Safe-Channel”
Transposer SCR.gpfav (1.6 KB)
I think, either way is viable, with or without scripting… it’s just one possibility more (endless +1) 