that seems like a lot of work but should be doable. At minimum you’d need something in the middle to measure the tension and then react accordingly, like a load cell (I think this is what the MWA scanners use).
this is why we’re using the tecknic motors for feed and takeup. they’re designed to do things like hold the film in position, with two motors working against one another. then the third motor (the capstan) handles actually moving the film. In this way, feed and takeup just do one thing. regardless of the amount of film on either side, the motors will hold the film where it is, until the capstan makes it move. If one side detects slack, it automatically takes that slack up. If there’s too much slack (say the film broke, or a reel ended and it’s unthreaded), the motor simple stops. You set the parameters for these actions in their software, it gets uploaded to the motor, and lives in its firmware. Very much set-and-forget.
The low budget way to do the tension sensing would be a dancer arm and a potentiometer. But if you’re looking for something cool, I think you should check out load cells.