Glad you're having fun with that Mike
Don't know if you could benefit from such a thing, but I did have a device that recorded the elapsed time between ignition events on a particular cylinder via an inductive pickup. With that data and the weight of the vehicle, Cd, frontal area, ect, it would plot out a real torque curve along with hp that looked identical to a real dyno chart, and was printable. The device I found to be impressively accurate from run to run at a +/- 1.5 hp variation.
The device was great at finding that last drop of hp and shifting torque curves around as well as recording elapsed time between transient rpm changes during a pull. That was an awesome piece until I outgrew it and no longer had traction below 60 mph. The temptation to do testing in a higher gear was enough that I gave it to a friend of mine in NY to prevent possible litigation.
If you think you could use somehting like that for testing I could probably get him to ship it back for a while. You would just have to be consistent with your throttle seeing as you'd be running it at partial throttle, but it would probably give you your power reading at max leanburn