Further info: the existing Pianoteq
https://www.pianoteq.com/ (I misspelled the software originally) is an independent, standalone setup having it's own computer, audio system - amp/speakers as well as it has it's own keyboard. There's no issue using a MIDI output from the organ console to play the piano, the issue is controlling HW Choir manual. The plan is to be able to add the Pianoteq to the Choir manual (registration) and be able turn off (cancel) the Choir manual's organ sound (piano only) and be able to return back to the same Choir manual's registration. In other words just as would be the case with a reversible.
The previous thinking was to do this within Hauptwerk. Re-thinking, the seemingly simplest solution is to provide a control (piston) to control (on/off) of the Choir manual's audio (output) with a second piston to control the MIDI output to the Pianoteq setup. I see that as a technically possible solution, but wonder how user (organist) friendly that would be.
In place of using pistons, we could use (stop) drawknobs or toe pistons for control.
A 3 way switch ie: organ only, piano only or both? The KISS principle?
Rgds,
Ed