you cannot use an Arduino Uno as an ISP programmer because the optiboot bootloader does not support this sketch.
Yes, I saw that. But it doesn't make any sense. A sketch does not normally require any "support" from the bootloader.
I envision a "opticlone" sketch, with a 6-pin cable coming off an arduino. Plug it into some other Arduino's ISP port, hit the reset button, and it would automatically power up the slabe, figure out what CPU was there, load up the appropriate bootloader, change the fuses, and power it down again (no need to run AVRdude on a host at all...) (Version control of the optiboot(s) in use might be ... tricky.)