Ok, the T.RJ12 USB adapter came yesterday, so I had a quick go with my T500RS pedals.
As expected, they work perfectly, since they are of course 100% identical to T3PA-Pro pedals.
Now Thrustmaster pedals deliver 1024 steps of resolution in total, which means if the potentiometers were perfect they would show raw values of 0 to 1023. Mine clearly need a small amount of deadzone, but much less than Thrusmaster provides by default.
Looking at the picture below, you can see that my pedals show a deadzone requirement of under 10 steps at the start of throw (as you can see they register 6, 7 and 0 respectively, there is a small fluctuation but not more than 1-2 steps). They also show a deadzone requirement of around the same at the end of travel (they register up to around 1016-1018).
The stock deadzones are 102 steps at both ends, but I could see anyone who dislikes the size of the stock deadzone getting one of these adapters to be able to use the calibration tool, just to reduce their deadzone from 102 steps down to ~20 steps. For some people this would actually be a big upgrade.
Here's my screenshot of the Calibration tool for reference:
P.S. The adapter comes with a "car/plane" switch. Makes me think that Thrustmaster are planning to allow use of this adapter with rudder pedals, perhaps in a new flight/space sim product. There's no information in the manual about what the plane switch could be used for at this time but I would guess it's for "combined pedals" as needed for rudder.