I'm posting my findings after solving this, just in case anyone finds this later.
The Qualcomm Mdm9615 (and later I've tried on a 9628) don't have a real serial port for the nmea traces. They use a proprietary Qualcomm API for communication between the 2 cores of the device.
One such call allows you to subscribe to NMEA traces, so you basically get a function called every time a new trace is received. Many device manufacturers run a daemon somewhere which subscribes to this API and posts the traces to a tty device, basically emulating a serial port.
Gpsd will not work with this emulated tty, as it requires a real serial port, so I looked for another way.
I wrote a small daemon to subscribe to the API (so I get a callback each time a nmea trace is generated), and then send each nmea trace over a UDP packet to a port on localhost. Then I set GPS to listen to that port, and everything started working right away.
Thanks for your help!