The idea behind mapping the 40/80 columns key to F8 (not F7!) was that the locking mechanism can be removed, so you have a separate F8 key. Same goes for the Ascii/DIN key: The locking mechanism should be removed for "everyday operation".
It's true that some C128 keys can't be 100% mapped to the Vice key map, but that's because it would interfere with the C64/VIC20 mapping.
We use a C64 during production to program the EEPROM chips on Keyrah: A special userport cable makes the connection to the jumper header (jumpers need to be removed of course), and the three chips are programmed in one go. I could think of a public version of this program, along with a setting for "C128" that sends the exact key codes that Vice is expecting for a C128 keyboard. One of the mappings will have to go - either a country-specific mapping, or the "PC-geared" mapping.
It's ok, I'll build something with an Atmega32u4 instead. If nobody else has asked for this, there's no need to accommodate me. I'll use the Keyrah for something else. Thanks anyway!