I don't think VooDooDoc is talking about a timed set up, but rather one that automatically fogs when humidity falls beneath a certain point. It would work just like a thermostat, where as a thermostat may be set at 90 degrees (on/off models heat up to around 92 then cool until 88ish and turn back on, good models change the power that goes to the mat), the hygrostat(?) would be set to a certain percentage of humidity and either replicate the on/off thermostat or maybe have a small amount of humidity normally but scales up with lower measurement. I like your idea, I just don't know if anyone makes anything like it. I might try to make one if I can't find one ha.
EDIT: Found one!
http://www.quiko.gr/D2C5B087.en.aspx
More:
http://www.dnjreptilestore.com/ht10.html
http://www.ebay.com/itm/EA-0401-Rept...-/260866733459
http://www.ebay.com/itm/30-OFF-110V-...eece30d&_uhb=1
http://www.ebay.com/itm/Reptile-Hygr...1#ht_500wt_926