But more importantly: if your miner had supported the target - which has been standard since getwork was created - you wouldn't have these rejects at all.
By the way, something occurred today: can variable difficulty be used to combat withholding attacks? The higher the share difficulty, the higher the cost of the withholding attack -- the "found a block" shares represent a larger fraction of the submitted shares. Could even be automated: if the pool sees a suspiciously low fraction of block-found shares from a miner, jack up their difficulty.