How can we facilitate flipping of images for RTL scripts in a clean fashion — ideally without adding otherwise unneeded mark-up, or relying on solutions that push this up the chain on the systems level (e.g. ResourceLoader, which may go away w/o us being in control / are unavailable e.g. in our cherished dev mode)?

This was left out of T233791 as, given the current asset, it is not strictly required and warrants a sustainable general solution.
Termbox solves this through CSS referencing a (redundant) attribute on a DOM element under our control.
ResourceLoader offers - somewhat "magic" - use of alternative file names.
Is there a better way?