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.
TIL!
But to be honest, that was also pretty much before standardization. I have never seen anyone use Meta for the Alt key.