Can difficulty target by exploited by miner with proprietary computer?
No, for two reasons:


His advantage is always the same. If he has 50% of the mining power, he makes 50% of the revenue when he's mining and 0% when he's not. The difficulty does not change this.
In the on/off scheme, when he's mining, blocks are being found twice as quickly. So the difficulty adjustment comes twice as fast. When he stops mining, he has to wait twice as long for the difficulty to drop, so he's mining less than half the time. By forcing the difficulty up and then not mining, he has to wait an awfully long time before it comes back down -- during all that time, his mining hardware is depreciating.


I am assuming that the only thing he does is mine or not mine. I'm ignoring the possibility that he attempts to force reorganizations or does other "bad" things.