Using USB data lines as GPIO

I would like to connect a custom device to the USB connector by (mis-)using the data lines from the USB connector. I have made a splitter cable so that I can power the Photon and also connect something to the two data lines (PB14 and PB15).
Unfortunately I am stuck at setting up the peripheral, I have the code to set up the right pins as output, but it doesn’t seem to work. I suspect that I would need to disable the USB functionality, but I have not found the right way to do it.
I only required this to work on the Photon, so I don’t necessarily require a proper HAL/Wiring code, just anything that works will do… Any clues in the right direction are appreciated.

@satishgn, @mdma, just pinging you because I suspect you guys to know the answer…