Allegedly, on or about 23 August 2016, Tom Horsley sent:
I mean boot the livecd ISO file directly off hard disk.
Surely it ought to be possible. GRUB can do a PXE boot, that's just the OS tree from some other source. Perhaps there might be an intervening step that mounts the ISO, but lets you chain into it. Virtual computing boots off an installation in a container.
Probably ISO files aren't going to be the best installation download in the future. Optical drives are being eliminated in Macs, PCs will probably lose them soon enough. The demand for blank DVDs has gone down now that movie piracy has changed tack. The future is probably a bootcode and file archive that's not specific to any kind of media. Of course it would help if PC hardware wasn't made to pander to Microsoft's demands, but for their users' needs.