In proposal I mentioned, shield blocking in the incorrect direction would make you more susceptible to being "spammed", so this would probably result in more opportunities to spam, not less.
The only time most players are going to be missing blocks with any regularity is in a large group fight, where I feel the extra stun would largely hinder the primary role of a shielder.
I would love increased shield damage for missed blocks, and decreased (or completely nullified) shield damage for correct blocks. I've suggested it before a few times. People seemed interested, but it would require a significant investment of time and effort to implement properly (with visual and auditory feedback). I don't imagine we're going to see anything of that scale this late into the life of the mod.
I even made a suggestion on the
TW forums a while back, but I doubt anything will ever come of it.
Only problem I have is how to know if you've stunned them or not. From my experience with block stun, it's just not very good without the proper amount of cues.
Having game mechanics without proper feedback is always lousy. Block stun, hold bonus, and speed bonus are the most significant examples I can think of. Everybody grows accustomed to them eventually, but it takes far longer than it would with proper cues.