Connect everything to the drive and the HxC except DS0. Then use a SPDT switch to route DS0 to either the floppy mechanism or the HxC.
You can easily modify a two drive cable that is used with a PC.. Straighten the twist and cut the DS0 wire from before and after the middle connector.. Then just connect your switch there.. Use longer wires to make locating the switch easier.
If you want to go further, you could make it so that the HxC and physical drive can alternate between DF0 and DF1, but I'll leave that as an excercise to you. Hint: unless you have a big box Amiga, you will need the little circuit board from an external drive.. Or alternately you can recreate an external drive's circuit yourself onto a piece of vero board.
