I was actually working on my own J1850 VPW implementation with my '99 Suburban, but am in the process of finding a PWM vehicle to use to consolidate the two into one library. I haven’t done any PWM work yet but I do have a lot of the SAE documents for the implementation, including the full 4 page writeup on the protocols. If I get any work done on the PWM side, I’ll let you know.
Edit: side note; I was considering using my Tech2 to possibly add some support for the extended PIDs I can access. That said, as far as I know the PIDs are/can be different for each different vehicle or model year. I’ll see once I get to that point. As of now I’ve been having trouble communicating with my trucks’ PCM due to some ground issues that I’m still hunting down. (darn project vehicles!)