Here's a video showing the internals of the Q25 hack, and demonstrating some sounds on MIDI channel 1 (instruments: piano,...) and MIDI channel 10 (drums).
How was it done ?
Opening the Q25, I found a central controller board, and there was one test point marked 'TX' on the pcb.
I tapped from there to the breakout board's MIDI in (see photo).
Then added a reset chip to the S1 breakout, and connected S1 to Q25 ground and power supply voltage (connected after diode on the brown pcb in the Q25). Some tape is sufficient to hold it all in place.
Drilled a hole in the casing to add a 3.5mm stereo jack. Connected this to the S1 breakout (Left, Right and Analog ground)
And... done ! the Q25 is turned into a Synth with stereo out !