I dropped a few words: it should say "mountable file system within the executable"; in other words, an ELF or PE32 section that contains a read-only file system image.  FAT12, the MS-DOS format on floppy disks, would be reasonable and easy to handle; I discussed it in email with Lassi a while back.
See https://bitbucket.org/cowan/r7rs-wg1-infra/src/default/FatArch.md for the FAT12 formt and a partial Scheme API.

On Sun, Oct 13, 2019 at 12:53 AM Arthur A. Gleckler <xxxxxx@speechcode.com> wrote:
On Sat, Oct 12, 2019, 9:23 PM John Cowan <xxxxxx@ccil.org> wrote:
There's been a discussion recently on the OSI mailing lists of including the source code to GPLed programs as a mountable file system executable inside the ELF image.  The general feeling was that people wouldn't be inclined to trust such tools, since they would be only rarely used.

That's an interesting idea, although I wouldn't trust it without a way to verify that the executable code matched the source code.

What exactly does the "executable" in "mountable file system executable" mean, by the way?  I've seen that term used in Linux start-up logs, but I haven't found a definition.