Hey @dagar, thanks!
I actually want to use the RTCM inject feature of Qgroundcontrol/PX4, using an InertialSense on the GCS, and a second InertialSense in the air. It looks to me like the driver is duplicated in both, so I figured that it would make sense to just add it to the driver layer.
For starters, I just want to use it as a low-cost RTK GPS, but I've thought about bypassing the estimator and supplying full-state estimates as you suggested. That seems like a lot more than I'm willing to bite off right now, because I'm worried that I'm going to need the SPI interface to get sufficient throughput for that kind of high-rate data, but I honestly haven't looked at it too much.
Are there any extra considerations I have to take for RTK beyond just extending the GPS driver?