I’m sorry for the misinformation - I was sure we had Serial2 since I’d not heard otherwise, but as BDub mentions, the 2nd UART goes to the LED pins. These pins are broken out on the underside of the Photon, so could be accessed and used in your project.
A future version of the firmware will make it possible to disable the onboard LED and allow you to route the LED signals to other pins, allowing the original LED pins to be used for Serial2.
I hope that this is workable for you and gives you a route forward.