This isnt possible in game rn due to heat being measured in thrust rather than after burner (it is somewhat but its a multiplier and im not even sure its specific per jet). On the redtop it has this implemented and the only jets possible to get head on shots from further out than 1km (when within 1km it gets hard to see exactly where it starts so sub 1km is all the same) are the F-106 which you can lock from 1.6km and the yack 38 which ive gotten from 3km before but im not entirely sure why its so easy when the harrier is quite hard. I think weve managed to lock an EFT frontally from something stupid like 3-4km but thats the odd one out.
If gaijin ever decide to revamp how ir sig works in game then i think we could have a more balanced approach to this, as missiles like the 9l would finally have their decent flare resistance while missiles that had limited all aspect capability would finally be able to lock onto the after burner ploom and skin heating rather than rely on the heat of the engine along. Would also fix jets like the F-5 being sub zero.