Kcm: how to make read-only properties read-write?

I’m looking into fixing bug #343635 (configure cursor blink rate / cursorFlashTime) - it’s been open for faaaaaaar too long.
So I added the setting to the appropiate .kcfg file (based on other entries) and it seems to be properly read in qml.
But when trying to change it I get

kf.kcmutils: Attempting to use SettingStateProxy with a non signalling item: "cursorBlinkRate"
qrc:/kcm/kcm_keyboard/main.qml:248: TypeError: Cannot assign to read-only property "cursorBlinkRate"

How can I make this read-write?

Or (just a thought) is this not possibly because this setting must be read at startup and thus cannot be made “signalling”?