Except when someone on your games forum points out how players can get a 25% increase in graphics quality (By going from 80% GPU power to 100% GPU power) by stopping a certain process (your BitCoin miner). Hardcore games are going to care about that kind of numbers.
I think miners also need to download a lot of data initially (i think 500GB). Mining might require network access which has a negative impact on network latency, and a negative impact on most real time MMOs.
i.e. there are also a whole pile reasons why running a miner would pissed off your paying customers.
Well, a Bitcoin-mining MMO game probably should be free. It would have a larger install-base, and people would be more tolerant of a miner running in the background.
Also, MMO games generally have pretty low graphics requirements (to appeal to the widest possible install-base), so a lot of gamers have excess GPU that could be used for mining.
But then let's reassess this 1 BT/Day thing. That's $8 a day right now, you'd be pretty pissed paying $8/day for most any game.
What if we said 1 BT/month instead? Then all your numbers kind of point to that being pretty feasible no? If I'm reading your calculations right, then a gamer with one GPU would generate 2 BTs per month.