Apparently, importing ~/.config/kglobalshortcutsrc is enough for the shortcut to show up in the System Settings, but not enough to load the shortcut. Disabling, applying and re-enabling and applying again also doesn’t help, and neither does re-assigning to the same shortcut, which resulted in this lovely error dialog.
Computer keyboards have been around since at least the 70s and the Windows key only started appearing on keyboards with the release of Windows 95. I am a child of the 80s and my first 3 or 4 computers had keyboards with no Windows key.
TIL!
But to be honest, that was also pretty much before standardization. I have never seen anyone use Meta for the Alt key.