Unfortunately, "Save All" doesn't appear in the menus–it's a command that you can access via a keyboard shortcut (what I'm doing) or the "ribbon," which I don't think KM can access (which also means I can't make a shortcut to the command in the Mac OS keyboard settings, as far as I can tell.)
I did find this in this forum:
Which only auto-saves the frontmost document, not exactly what I want, but a functional workaround.