I found a System 0.97/Finder 1.0 image that works, as well as a French System 1.1 Finder 1.1, and the 400K version of Dark Castle. All run fine as emulated 400K disks under Floppy Emu on the Mac 512K. I don't know what's up with the US System 1.1/Finder 1.1 image, but I'm not going to worry about it any further.

Looks like Floppy Emu is now officially working on these 64K ROM Macs! Now, for 1.44MB floppy emulation support...