Okay, so - a solution that works well for us in #bevy with Orthographic cameras, since we generally design for 1080p and scale + letter/pillarbox as required: https://pastebin.com/S51zTP3C
Basically force `scaling_mode` to `Fixed {width: 1920, height: 1080}` and `scale` to `1.0`
above solution is incomplete, will not play nice on ratios wider than 16:9 or resolutions smaller than 1080p, but works okay on various systems/displays my coworkers and I tested on.
...left as an exercise for the reader