I wonder how viable doing this openly would be as a free-to-play business model? Being upfront with the user: "we're not going to show you any ads and you won't have to pay anything for extra content, but we'll be using a tiny proportion of your CPU time to mine cryptocurrency to pay for the development costs of this app."
It was done secretly in the recent past, and apparently while it lasted (weeks, i think?) it brought in something like $6k. It helps that most gamers have fairly powerful graphics cards. If you mapped bitcoins mined to microtransaction currency (for in-game rewards) most players would love it.
They claim the value was almost USD$4,000 over about two weeks, although we don't really know how many users were affected and for how long. This doesn't seem like an extraordinary amount, though, especially if we're talking about using mobile phones instead of high-end gaming PCs.
[1] http://www.pcgamer.com/2013/05/01/esea-accidentally-release-...