You could use the Maestro to generate RC signals, which the jrk can use as an input to control a motor. Those RC signals can control a motor with or without feedback. The jrk has a single feedback pin, and it can use the frequency from speed measuring devices, like a tachometer, or analog voltages from something like a potentiometer. So, if you want to use feedback, then you would have to connect something like a tachometer or potentiometer to the output of your brushed motor. You can learn more about the jrk’s feedback under the “Feedback Options” section of its user’s guide, which you can find under the “Resources” tab of its product page.
The interface you described “that would convert the RC signal from the maestro to an analog signal to drive the FB pin on the Jrk.” would not work. A key aspect of a closed loop control system is to use measurements of the actual output (which in practice is different from the commanded output) to update the input. You can read more about PID controllers like the one used on the jrk in this Wikipedia article.
Both the Tic and jrk motor controllers can be controlled with hobby RC signals that the Maestro can generate. If you tell me more about your application, I might be able to help you decide if a stepper motor or brushed DC motor is more appropriate.