It looks like the Ranco Model J10-810 may do the trick. The description isn't very good but it looks like it compares the relative humidity between the outside air and the attic air and turns on the fan when the humidity differential is either 5% or 15%. I assume that it has to have an outdoor sensor as well as the sensor in the control, but the product info doesn't talk about a separate sensor. I e-mailed the company for more info. http://www.ranco.invensys.com/solutionspdfs/Residential%20HVAC/Humidity%20Controls/hvachumi.pdf
The humidistat that comes with an attic fan and the typical add-on humidistats are pretty worthless for this application.
By the way, this demonstrates yet another of the many reasons why passive attic ventilation is so much better than powered ventilation. Unfortunately I couldn't use passive because my house has no eaves (and therefore no soffits) and there is not enough space for drip edge vents. Also, ridge vents aren't practical on my old slate roof. Powered ventilation was the only way to go.