It's working (MISTer) but I think it's the same on MIST.
Just load with the EP commands as described above.
To switch the disk's do the following:
Hold F7 and press Space. My second LED on the keyboard starts flashing. You can release all keys now.
Now press the desired Disk Number 0 = Disk 1, 1 = Disk 2, .....
It doesn't matter how long you press the keys. Just do it as above and it works directly.
Two beer or not two beer? - Shakesbeer.