Thanks! This seems like something that Valve had intentionally disabled, so once they become aware that some people actually want it, I assume they will add an easy setting to re-enable it.

But in the mean time, so people don’t have to visit Reddit, here is the relevant bit from that post:

The Actual Fix via Konsole:

Once you know the mouse is paired correctly, and see that it works (just the pointer is not visible) open Konsole and run this single command to force KWin to render the pointer icon over the Gamescope layer:

mkdir -p \~/.config/environment.d && echo "KWIN\_FORCE\_SW\_CURSOR=1" >> \~/.config/environment.d/kwin\_cursor.conf

Make sure to restart Desktop Mode (or reboot the headset) after running this for the changes to take effect.