No it won't work that way you think it will. Think of the dimmer in terms of percentages...if you turn the dimmer down half way then it is only allowing half the power to get through.

(as an example we will use degrees)...say your thermostat is putting out 100 degrees this gets to the dimmer you have set at anything less then full (since the dimmer is power management not temp management) will decrease this 100 degrees down some percentage. If you increase the temp to say 110 the dimmer will still decrease the temp getting to the tub so in essence you aren't creating a safety you are having to increase the thermostat higher than necessary to compensate for what the dimmer is removing.

The only way to do what you want is with 2 thermostats (but the backup one can be a cheaper one...but it must measure the temps independent of the main thermostat otherwise how does it know the temp is too high)

Hope this helps