It works! Direct link here:
http://www.otenko.com/macintosh/Presto-PPC-8.5.sit
I'm not sure where I went wrong last time I tried the 'gusd' mod, because using the same System Folder again now works. Could be I was a goof and tried to boot without switching to 601 mode...
Anyhow, I copied an existing 8.6 System Folder from my TAM and added "Presto PPC 8.5 Enabler" to the root directory, and the Quadra/601 happily booted to it. The "Presto PPC ID" extension changes the gestalt to 112 (7100/66) so the 8.5+ installers will run, same as Wish I Were or MachID Wannabe, and it's not needed to boot from the installed System.
Thanks for bringing it up; I had given up searching before but I'm glad to have it now!