Dimmer as backup to Thermostat?

[ctrlfreq]

Both of these devices (Dimmer and Tstat) are current limiting devices.

Yes, but one has internal circuitry designed to operate within a specific power range to limit the current.

And yes, the t-stat will increase power to compensate. Does it do this by reducing resistance?

Depends on the model of t-stat, but most simply switch the power on and off until the temperature is in range. In either case you can damage the circuitry that does the thermo-regulation, whether switched or metered, by under powering it.

The t-stat running at something like 98% open to achieve a temp of 92 deg. If it were to fail completely, then the dimmer would be getting 100% of the power

Most will shut down in case of a failure, an important feature to note when purchasing a thermostat. Also, most thermostats don't meter the power amount, but rather are "set" for target temperature ranges, which are measured for by the probe.