So no, it is not possible to use the GPIO Hackerboard with the XL. 

I just got a Prusa XL with 5 toolheads and I was hoping to attach my GPIO "Hackerboard" to it to do nice timelapses with my existing MK4s setup but I'm struggling to find a fitting port on the XL.

Reviewing the electronics documentation it looks like that the xl has not I2C port and also the hackerboards documentation does not state that the board is intended to be used with the XL.

However, as somebody who is designing PCBs on professional leval I'm not afraid of trying something. 

While reviewing the available schematics of the XL I found an unpopulated connecot footprint (P11, mid-left section of schematic) in the schematic of the LCD-PCB (FDM-XL-MKx-xLCD-29.pdf) that provides access to the required pins(3.3V, GND, SDA, SCL). So I should be able to connect the hacker board phyiscally to the XL electronics soldering some wires to those pads. 

However, that would required on software side that I can use the same GCODE commands with the XL as with my MK4.Does anyboady know if this is possible? Otherwise I don't need start tinkering with the hardware at all. 

Chears,

Marco