I've just tried the pokey core on the pure digital chips. It works surprisingly well!
For some reason I'd remembered the keyboard 4051s were CMOS, so I didn't think the 3.3V output would work being <3.7v (http://www.interfacebus.com/voltage_threshold.html). However it works just fine, though perhaps not on all boards. Are the 4051s really CMOS levels?
The audio output is the raw sigma delta output, with no filter. However it sounds fine, perhaps relying on the normal audio filter on the main board.
Paddles do not have the correct voltage threshold, which is expected.
Less expected though is that paddle 0 works ok while paddle 1 completely fails. This one is weird. I'd connected multiple MAX10 GPIO outputs together so I could use multiple weak pull up resistors in parallel as a stronger pull-up! Paddle 1 happens to be on one of these pins. If I set them all to high impedance its pulled up to ~3v. Individual max10 pins work, together they pull high! Seems to be an interaction with multiple GPIOs at high impedance and the quick switches. I checked this on multiple boards to be sure. Weird...