@Eric-Katzfey Thanks for the tip!
The serial cable seemed to be OK, but updating the system to 0.9.5 via a fastboot recovery seemed to solve the problem. I think this issue relates to https://forum.modalai.com/topic/525/voxl-adb-or-wi-fi-no-longer-communicating but with the added symptom that I could ssh into the VOXL board over wifi as well as ADB in, but that the mavlink connection really seemed troublesome and QGC wasn't connecting over wifi. I just used VOXL Platform 0.9.5 for my update and that seemed to save things. (for those reading, make sure your extrinsics file is saved over on the /data/modalai partition! Mine was over there through good luck rather than foresight and confirmation on my part...)