Does this work? Players can normally only play cards from their hands. A card like Yawgmoth's Will specifies that graveyard cards may be played as though they were in hand.
Perhaps a better versiion would be one that counters all cards not cast from hand. Yes, this is slightly weaker, but much more clear rules-wise. And, at this rate, why not just give the WW bear Flash? White Bears have gotten strong lately -- I don't think this will be above the generally accepted power level.
With the white bear, giving it flash leads me towards making it a one-shot "counter target spell that wasn't played from its owner's hand" effect.. but at that point, it's no longer white.