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)

its very simple i think. create a pool or workers on an existing pool and award gametime (or something of worth for the game) for generated shares or btc.
not much different than normal mining and then paying for the game, just that its more convienent for the player. best works with some kind of automated setup for a gpu miner.