Thanks for the tip, and the email! I've ordered a scan tool that can read SRS codes (since my current one can't), it should be here by Monday. Hopefully it will tell me the code is for the driver's side seat position sensor - I can't imagine what else it could be.
For anyone else who comes across this thread in the future, vehicle has manual seats, all stock, no modifications. I tested it probably 20 times last night that the light operates exactly as I described in my video - airbag circuit deactivates when no one is sitting in the chair (light comes on), but works normally when someone is sitting in the chair (light goes off). If I get out and push down with even moderate pressure on the center of the seat, the light will turn off. Moving the seat forward & back doesn't seem to have any affect, although with my height (I'm 6'3, 190 lbs.) it's uncomfortable for me to try to sit with it anywhere other than slid all the way back.
Frankly I don't see what the problem is with the airbag circuit deactivating when no one is sitting in the seat (and it's never failed an inspection in all the years I've owned the vehicle), but still worth looking into. Here's some info cart7881 kindly provided:
The seat track includes a metal bracket that shunts the SPS magnetic circuit creating 2 states of seat position. The shunted state represents a rearward seat position. The non-shunted state represents a forward position. The SPS provides 2 current ranges, one range for the shunted state and a second range for a non-shunted state. These 2 states are inputs to the SDM. State 1 (shunted) being the rearward threshold and state 2 (non-shunted) being the forward threshold. When the SDM receives input from a SPS that state 1 threshold is reached (seat is rearward) the SDM will not disable stage 2 deployment, if required by the deployment sensors. When state 2 threshold is reached (seat is forward) the SDM will disable stage 2 deployment on the side the seat is forward. The SDM monitors the SPS circuit and if a fault is detected the SDM will set codes B0083 or B0084 and defaults to disabling stage 2 frontal deployment.
Fingers crossed that this is my problem!