I think a barometer of advice quality would be useful to many of us, but obviously people self proclaiming themselves mPUA's would be a nightmare.

On the other hand, what if we had a simple number as flair, (upvotes - downvotes) only gained/lost on this subreddit.

Yes, it might create a bias against newer sedditors, but maybe it's not a bad things to favour people who've been more active until another member takes a bit of time to build up upvotes from contributing solid content.

If a karma counter is a good enough working model to base our view of individual posts on it, could we not simply extrapolate that to an entire user?

It wouldn't be perfect, but theoretically it would be as good as the system we already use, but give a wider range of information.