I'm not sure I follow what you tried to say.

Going on flat road, you have a "medium" gear selected. So your constant RPM * constant torque give you a "medium" speed where lots of the power is consumed to overcome the wind factor and a quite small part of the power overcomes other losses.

Going uphill, you have a "light" gear selected. So your constant RPM * constant torque gives you a "low" speed where the absolute majority of the power is used to accumulate potential energy, with hardly any wind resistance to overcome.

Going downhill, you have a "heavy" gear selected. The wind drag to overcome is tremendous, but you get lots of extra power from the "consumption" of potential energy. This extra power not visibile in the torque*RPM measurement. But not really relevant either, if the goal is to measure the amount of work the rider does. That power was already accounted for during the uphill ride.

So in the end, you don't suffer the problem that a constant power applied gives a constant acceleration and hence a constant increase in RPM. You will quickly reach a balance where drag + potential energy matches the riders efforts.

If anyone would think it interesting to figure out amount of power that was accumulated/consumed when climbing/going downhill, the rider+bike weight can be measured before start and a height sensor can be used. Normally a simple air pressure sensor works quite well since the air pressure isn't changing so fast from shifts in weather. GPS sensors often have significant problems to measure height differences - they are designed for precise position in latitude + longitude - not in height.

Anyone - commercial solutions (lots of them) are very expensive so there are lots of reasons for attempting DIY variants.

I wonder if anyone have cracked the protocol Polar used for their chain sensor solution - it receives the data using two electrodes on the pulse meter. Would be nice if I could feed data to these electrodes and have the software covered :)

Anyway - I do have a program that takes power info from the indoor cycle simulator and modifies the Polar pulse file with power information.