Good news! I got the Apple II cable from IEC, and did a continuity test to verify that it has all the necessary connections between the DB-19 and IDC-20 ends. I was a little worried, since it seems there's one pair of connections required for the Mac that are just unused pins on the Apple II, but the cable has them connected as needed for the Mac. Woohoo! So now there are more good options for connecting the floppy emulator.