If you want to do it in a 100% correct way, you need to use some sort of temperature texture for all materials. For example Arma 3 and VBS have a quite complex way of doing it: Customer Portal | BISim
Edit: one way would be to have a master material and then interpolate between some emissive texture set and the “normal” material textures. Then drive the interpolation by some parameter that can be changed in code. So when the thermal imaging device (or whatever you have) is turned on for the view, the materials switch to showing the emissive temperature texture only.