It wouldn't necessarily be very strong but it could become very frustrating... imagine a guy using such a stance constantly poking you behind his eternal shield... In any case, I find it extremely unlikely Taleworlds would add such a feature to the game.Well that dude posted a new shield he found in the game with the notch for spears so I'd assume it's something they've at least thought of. Maybe if the mechanic was limited to these types of shields then you could balance around that, like making the shield itself weaker. Seems like it'd be awesome and would make spears in formation good.
I don't think it'd be necessarily unbalanced for MP either as the spear is extremely lacklustre in it's current state. Shields already break in a few hits from 2handed weapons, and if the mechanic was tied in to the spear-bracing system I think it would be perfectly fine.