Or can this be achieved with a regular distro?

Use case: non-tech user can't burn ISO to pendrive to fix after critical crash requires reinstall. Can't guard against nuking the OS either because they're learning.

So I'm imagining a "repair" entry in the bootloader that just does that the external media would do, meaning the installation files are always somewhere on the disk. Dedicating 4gb disk space for the convenience is very okay.