@syl solved this with definable auto-tagging in his Docus plugin. I suspect someone could pull that function out to create auto-tagging if he didn’t mind.
from my perspective the most obvious solution would be for a post to be auto-tagged with the tag(s) that the user is currently on (tag-page, or post). This makes it obvious visually to the user how the feature works if they need to change it, and in most cases it will probably be correct as they will be replying to something with the tags they are replying to.