DC motor as a generator as a torque limiter?
I think your basic idea is OK, but the pot idea is not very good. The problem is that you have to take the position signal and differentiate it to get speed, and noise will be a big problem.

What you need is a rotary rate sensor, and there are a variety of these. An obvious choice would be a tachometer. DC motors can be found with built-in tachometers specifically to make regulating rotation rate simple. Encoders, particularly optical encoders, are cheap these days, and your job is made easier by the fact that you don't need an absolute encoder - incremental will work just fine for this application. There are other, more exotic sensors such as synchros and resolvers, as well.

What you do is to make a feedback loop that compares the rotational velocity to a desired value and adjusts the motor voltage accordingly. Depending on how ambitious you are for your loop design, you can adjust for a wide range of speeds and loads with a single circuit. And since you're interested in controlling speed rather than position, the feedback loop can be quite simple and still give good performance.