The PROG key is hardwired to reset the MCU into programmable mode. No matter what code you put on it, if it is held while flashing, it should work. If it does not, you can try holding the key and then re-plugging the keyboard. In other words, even if you put, say, Key_A on PROG, even if it does work as the A key during normal use, if you reset the serial port (which is done prior to flashing, to signal the board to enter programmable mode) and hold the key, it will still work as PROG. So you are free to remap it to anything, and it should still work.
If that still fails, if you take the enclosure off, there’s a small reset button somewhere. (On my prototype, its on the left side, beside the extension headers, but it may have moved somewhere else on the final hardware.)