Okay - so it was me trying to tag it incorrectly. With a ton of searching and debugging, found enough examples to get it to work.
For any users having similar problems going forward, the correct way to build the code in the Metadata is:
value: Lighting,Lighting.OnState,Lighting.Brightness
config:
dimmerMode: filterOn
The YAML editor just has to recognize it correctly to save. Even if you put it in incorrectly, it wouldn’t show a save, but WOULD propagate over to the metadata when doing an “openhab.metadata list” in the console.