I've not tried this, but my guess is it should work. In order to avoid excessive cycling on the keezer, I would use a ferm wrap heater like this, http://morebeer.com/view_product/166...ermWrap_Heater
, and I would place the temperature probe away from the fermenting carboy. It might also be beneficial to have the carboy on the freezer's hump that way it's elevated from the kegs. I would think that setup would be effective and minimize the amount of extra cycling.
I second temperature controlled fridge/freezer works good too