Hi,
I think your misunderstanding the dimmer concept.
It does not go between the thermostat and the wall - it goes between the thermostat and the heater.
The thermostat gets full line voltage at all times and would not be screwed up in any way by the above setup.
The idea is that if the thermostat jams full on the damage is limited by the throttling of the dimmer - meaning that while the tank will get over temp it should only be by a few degrees not 10 or 20.
dr del