SOLVED: I was close with number (3) above.
I just needed to create a folder called linux
-- and then put a bootloader file in that folder (I copied the grubx64.efi bootloader file from the ubuntu dir) --
to be able to put an os_linux.png file in the /boot/efi/EFI/refind/myicons folder, and have that image file show in the rEFInd menu.