Can conformal coating help against high-temperature environments if the chips heat up as well?
They don't want you to know that all plastics are thermal insulators unless they contain metallic particles like alumina oxide and would be grey in colour.

For the automotive market, spray or dip conformal coatings are an essential process. 

The epoxy used in IC's cases has better than average heat conduction in [W/m'K] but mainly it is selected for it's moisture seal resistance.

So how do you keep electronics cool in the engine compartment?  


Well not likely by radiation.  
More likely by baseplate conduction to the frame.