All credit to this thread for figuring out how to do this, I'm just documenting it more thoroughly.
Tested on Nova Air C with firmware 3.5 (2023-11-20).
Handwriting optimisation makes the pen usable in apps installed from the play store. By default it's only available for OneNote, Evernote and WPS -- they have a 'handwriting' tab in the optimisation settings, other apps don't -- but can be enabled for any app by editing the file /onyxconfig/eac_config as root.
Note: whenever you edit eac_config, you have to reboot the device for the changes to take effect. Just restarting the app isn't enough.