Dimmer as backup to Thermostat?

[ctrlfreq]

I think the idea for the rheostat being there was in case the thermostat shorted.

I can understand that, but the problem is, if the rheostat is inline with a thermostat, and the thermostat fails, with any decent thermostat, there will be no current on the line after the failure. If you're looking to have a true backup, it will need to be parallel to the t-stat circuit, and not in series.