Do you have an idea on what represent 20% of a normal CPU mining bitcoins for a whole day?
I guess it's something like 0,0001 BTC (today, not including future difficulty). Now, you don't run the whole day but only a few hours. Let say that you play 2,5hours/day and you play 3 days/week for 50weeks (nearly the whole year). That's 0,0015BTC/year.
If you have 10.000 players playing at this rate (and that's fairly optimistic), it would be something like 15BTC/year.
At this rate, do you think it worths the trouble?
Maybe, it's a lot more useful for games developers to put a bitcoin donation address on their website. Battle of Wesnoth is already on Flattr (I subscribed to them) and I believe they are making more money that way:https://flattr.com/thing/56371/The-Battle-for-Wesnoth
(But it would be a good idea to convince Battle of Wesnoth developers to accept donations)