TLDR: Meta+Ctrl+Alt+[Direction] (or a similar shortcut) should rotate the screen in the desired direction, regardless of whether the screen is locked.
Alternatively, Shortcuts — System Settings should allow shortcuts to be opted in to working when the screen is locked.
Using Shortcuts — System Settings, I set shortcuts to rotate the screen (which do not work when the screen is locked).
For example,
Meta+Ctrl+Alt+Left runs a bash script containing
#!/usr/bin/bash
kscreen-doctor output.1.rotation.left
and
Meta+Ctrl+Alt+Up runs a bash script containing
#!/usr/bin/bash
kscreen-doctor output.1.rotation.normal
Motivation: Physically rotating a laptop to view videos with a portrait aspect ratio. When [the screen is rotated, and the screen locks, and no way exists to rotate the screen when it is locked], I become slightly annoyed. I prefer ‘Landscape/normal’ mode when unlocking the screen.