Of course you can change the tags locally using Picard, and it will use your tagging settings and tagger script when saving. Plugins is another thing, there it depends on the plugin. If the plugin provides some tagger script or an action you can choose in the context menu, it operates just on your local data. Some plugins however affect the data loaded from MB, those plugins won't change your local data on their own.
What I mentioned elsewhere is that Picard is primarily meant to be a tagger using data from MusicBrainz, that's what it is designed and mostly used for. You might find other tools easier and more comfortable to use for fully manual tagging. And why not just add the releases to MB?