The unpopulated header on the left looks promising. Little work with a continuity tester revealed that the pins are connected via the bus buffer to parallel port input pins 13, 12, 11 and 10. (I don't know what the fifth pin is connected to.) Pin 10 is used for ESTOP, but pins 11-13 are unused. Each pin has a 4.5K pull-up resistor.
|Pin header soldered in and mono jacks for connecting the switches
I tested the inputs by shorting the connectors and checking the pin statuses in HAL meter. Next step is adding the actual limit switches.