No logic. You can connect only

RayeR писал(-а):
So there must be some switching logic between SuperIO and PS/2 connector.
No logic.
You can connect only one device - keyboard OR mouse.

RayeR писал(-а):
Does anybody tried a HW modification to the motherboard to make possible both KB+mouse?
I not tried, but IMHO your idea is real.
In your case MDAT and MCLK have pull-up resistor (8.2 kOhm, for example) to +3.3VDUAL. No other connections.
You must find this resistors, desolder it, connect MDAT and MCLK via series resistors (82 Ohm, for example) to pin 2 and 6 (for Y-cable splitter) or to new PS/2 connector, add pull-up resistors (8.2 kOhm, for example) to KB/MS power line and capacitors (180 pF, for example) to GND.