The base-drain Q1 and Q2 can of course be used. The problem is that the base-drain voltages to temperature is only approximately known. You would need to calibrate those, using external heating. As they can be be expected to be linear, you only need two or three temperature points for an decent approximation. You can also do a thermal energy balance and compare it to the electrical heat balance if you measure the input current and the voltage / current over external components, ie the resistors, opamps and external transistor. Most of the energy should be dissipated from the heater anyway, and the diode current is known....