Bottom bracket damage work-around
You could most likely also instead do the following: face the frame by 2.5mm... don't install a spacer under either cup.. install them directly into the frame. When installing the left side crank slide a spacer (1 1/8 inch headset spacer for example with on side pressing against left side crank and other side pressing against the rotating ring on the bearing cup) over the axle of the crankset before proceeding to install the left side crank arm. Make sure to choose a spacer with just enough thickness to ensure you can adjust it so there is no play on the bearings/crankset axle. I've done this on one of my mountainbikes and ridden it for well over two year without issue.
Using this method you have the entire bearing cup's thread length engaged in the frame even after facing off 2.5mm (or another amount for that matter).

Your suggestion of facing off only the called aluminium sounds like a good option to me. I believe the amount/total thickness of spacers underneath the bearing cups is limited because there needs to be a certain length of thread engaged to aquire the required strength. This would also explain why it differs between metal and plastic cups (since the latter have much stronger threads). This being said a 2.5mm spacer shouldn't cause any issues based on my experience. Perhaps you could consider tightening the bb cup on the faced side a little less (make sure it's properly snug though otherwise you'll ruin your threads most likely ... As is also the case when not tightening a pedal correctly and using the bike after) and installing (high strength?) Loctite or another thread locker to prevent the cup from loosening. .