I am currently designing a custom circuit using the P1. I think the P1s will be sent out sometime later this month so I am looking to get the PCBs made in time.
While hardware files are not available for the P1, you would be able to extract a lot of information from the Photon schematic since P1 is basically Photon, without the LEDs, buttons, voltage regulator, and legs. The Spark store says it comes pre-loaded with Spark OS so you would want to keep the same comm pins as Photon.
An example of USB connection (for firmware update/UART with the computer):
P1 would likely use PB15 as data+ and PB14 as data-, same as Photon.
PB14 is the port for OTG_HS_DM on STM205 and PB15 is the port for OTG_HS_DP. You can see this on page 50 of the the MCU datasheet:
These two ports are 5V tolerant, btw. I am not sure how to enable the internal pull-up resistors via software but I assume the Spark OS does that for you.
I hope this helps a little bit. I am also trying to figure out how to best design my P1 circuit.
They were just recently added!