If we really want custom hardware involved, the most economical solution would probably be a USB repeater that did the re-interpreting in hardware.
Took me a while to work out that the reason my pasted passwords would occasionally not work was the remapping of a couple of symbol keys. (I'm accustomed to a weird mashup of USA/UK layout, thanks mainly to Apple's non-standard 'UK' layout donating Americani?ms, but then useless prominent keys I swapped for something more helpful (backtick & pipe I think, off the top of my head, for things so useless that I literally never use them and don't know their names.))
I have thought about custom in-line hardware but that's honestly as complicated as QMK. Plus latency.
What I would love however is an inline monitor to see what keycodes I'm issuing. It gets...zany.
this is actually very simple to do with QMK if you have a board with an OLED display. it's also implemented in the default keymap for a bunch of the split ergo boards (Lily58 is one that has it)
Mac: Karabiner-Elements[3]
*nix: idk but should be plentiful ways
PC DOS: you’re probably out of luck