You're right, I haven't tried this. But my (unmentioned) approach was going to include using at least one fully programmable keyboard -- QMK or similar -- to send precisely the keycodes wanted.
It doesn't work that way. The USB HID usages (and PS/2 scancodes) are the same. There is one usage for the key at (say) D01, and the keyboard doesn't know or control whether it is "Q" or "A" or something else entirely. You cannot switch layouts on the keyboard, because the maps are on the host. Multiple keyboards with D01 engraved with different symbols will not actually vary anything, and all send the same thing across the wire.