I think that it would be a brilliant idea if as said previously by others, a cooldown was implemented so people couldn't just charge in crazily. Also it shouldn't do more damage than a kick unless shields were implemented with spikes, which would actually do damage. As long as its added with care, with thought as to how to keep it from making shield users indestructible then I wouldn't be against this!
EDIT: After searching a bit in the taleworlds forums, I have found that this mod has something close to what we want. Obviously I have no idea on how to pecure/create our own though.
http://forums.taleworlds.com/index.php/topic,127307.msg3070091.html#msg3070091Just ctrl-f and type "shield-bash" with the dash.