If you went for some sort of PLD, couldn't you then dispense with the PCF8574 IO expanders, and just do everything in the "PLD"?

Makes sense.

It just strikes me:
if you insert a (C)PLD as a "switch fabric" between the PCF8574 and the I/O connectors you will lose the bi-directional function of the pins. Anyhow that should not be a problem, Geert: you are not thinking of bringing the PCF8574 pins outside the board without a buffer, I hope.

If you pull it all into the (C)PLD you can make them bi-directional but make sure you use a (C)PLD that can withstand the stress of being connected to the outside environment.

Erik