Confirmed it looks like some sort of heater/coolant inlet/outlet, but I can't figure out what it is... :S
Here is another cylinder head that actually has the fitting, and this article shows a heater return pipe for a RWD 4A-C that looks like it would fit there... So maybe it is a crossover head? A deprecated heater fitting?
The VIN number lookup didn't work with my VIN number, despite the fact that the VIN on the build plate matches the one stamped into the body under the hood, so I have a feeling Australia threw together some off-cuts at some point. My hatch, for example, is designated an AE94 in the VIN number and an AE92R in the model number, despite the fact that E90 series ceased production in 1992 (except in Australia, where we continued production until 1994).
In my eyes this cased is closed! Thanks for the input!