An exhaust fan with a humidity sensor in the exhaust duct keyed to a cutoff switch should do the trick.

Yes, it is a simple thing in principle but, as with anything, the implementation could be difficult, and I suspect that there would be problems with a single humidity sensor in the exhaust duct. I would propose the above, in combination with several moisture detectors (such devices are commericially available), placed on the inside drum wall in various locations. The clothes would brush up against these detectors, which would have to be very sensitive.

Fine tuning of the algorithm would be required. Something like if no moisture detection has occured for such a period of time, then the clothes are sufficiently dry. The algorithm would be modified for different drum sizes, maybe different load sizes etc. Comments?

I believe that Maytag's Neptune Intellidry Dryer has this feature. (One result is that the drying time varies unpredictably.)