I have been planning the same and connecting the external HxC via a port on the back with a switch.
I might be all wrong here, but I was thinking of cutting the floppy cable open and soldering a second connector, then have a switch on the floppy power instead, when the switch is in one direction the power go to the internal floppy, when you flip it the other way the power go to the second floppy, same data cable but only one with power at once.
I think this would work but I have not tested.
