Sorry for the long delay, funny how time flies...
I still haven't made any progress on this due to work and home life conflicts, but it's still on my to-do list if anyone else doesn't get to it first.
The main challenges I see are that the Beaglebone Blue has a librobotics API, including system calls to get to PWM via the PRU, etc. That makes it pretty straightforward to port, but it's completely different from every other Linux platform.
On top of that, with Linux it probably doesn't make sense for PX4 to talk directly to the IMU, etc. chips. In the case of Beaglebone Blue in particular, that won't work at all for the PWM outputs since they're simulated with PRU firmware.
For the sensors, they're all accessible via librobotics or /dev/* in the usual Linux ways. Still not rocket science, but still different from how things seem to be done on NuttX targets.
Long story short, I think it's mostly just an exercise in stitching together all the lower-level stuff, similar to what was done for Navio but implemented very differently. The net result might improve the Navio port, too, if we can find common Linux-level APIs to exploit on the two platforms.