It has been resolved! Turns out there's a button in the middle of the right-most knob that makes it blow cold air.
I kid, I kid. Smart-ass answer aside... no, there's no fix for this. Since I've started driving back in the early 90's,I've been in cars where the cold setting is almost as good as AC, and others where it's only marginally warmer than outside air... but none have been as bad as the 86/ BRZ. I'm guessing it's just a quirk of design. Personally, I haven't noticed it getting better or worse over the years; I guess we just drew a bad hand with this design. Maybe the only way they could work out how to get power to the
rear wheels was by routing the exhaust through the air-ducts... who knows.