I don't deal with this sort of stuff much, but if the ability to use aux heat is controlled by a low-voltage circuit, you could break the aux heat enable from the thermostat probably easier than trying to control the high amperage of the heat coils themselves. So, do a search for a SPST NO timer switch. then, run the aux heat control line through it. It would only be closed when the timer allowed it to. Since it would be controlling low-voltage and amperage (i.e., the power to enable the contactor to supply the heat), it would be fairly easy and not use much energy in the process.