Yes! My floppy emulator now successfully boots the Xerox 820 by emulating an FM-formatted 8" floppy disk drive!
The hardware is a prototype Adafruit Floppsy and the code is not online yet as my last "aha" moment about correct FM encoding probably broke MFM encoding and I didn't want to do that.
I'll get it cleaned up and pushed to that open pull request in Adafruit_Floppy tomorrow with any luck.
(Neither MFM nor FM emulation support writing, and I don't think implementing that's on the table right now, so this is still much less capable than a gotek with flashfloppy.. but I'm still excited about it)