The prebuilt dimmers (e.g. walmart $10 mentioned above) can goto one or more devices if you plug in an extension cord.
The real limitation with the walmart model is that you can only have so many devices hooked up to it before you overload the dimmer (up to 300w worth).
For about the same money as the pre-built model you can build your own that will handle twice as much wattage (600w).
Really the walmart model is best for most small applications but I like to build things rather than buy sometimes. That and I wanted a dimmer with a ground plug for my 1500w ranco on/off thermostat.
For what you want you would need a thermostat (on/off or proportional) to shut the heat off at a certain temp and turn it back on again when it cools off. The dimmer basically just keeps the same temp difference.....not the same temp.
So if you have an 80 degree room and set the dimmer so that the hot spot is 90 degrees the dimmer essentially is set to +10 degrees.
So if the room heats up to 85 degrees then the hot spot will change to approximately 95 degrees if you don't change anything on the dimmer....same +10 degrees
Hope that makes sense