The only thing a timer could really do is just make it so that it's off longer, if you tie the two together the fan will still be cycling like crazy during the timer's "on" cycle. The problem is just like with a topoff, you have a setpoint and a tolerance and the smaller that tolerance is, the more on/off's you'll get in order to keep the feedback at the setpoint. Whereas with a larger tolerance the value can drift farther before the controller kicks it back on.
I am not sure if a dehumidistat exists that gives you a variable tolerance.
What you could do is just take the dehumistat out of the picture altogether and just use a timer (assuming you can find a timer with enough programmable settings and can handle the load and all that good stuff) and then you become the humidity meter and adjust the on/off cycles or durations. Sort of like dosing with a peristaltic pump and a timer.
Maybe that's a kooky idea, I dunno...