Web based mining - as solution for subsidize site owner, is not so good idea, really.
1) Lots of people using mobile devices, and they will be afraid by extra battery drain, even more than by adds.
2) Webmining, by itself, can not be source of income that we can call, anyhow significant. (thats most important part)
3) If (somehow), that idea will spread, users will start to block immediately, any suspected script.
4) That will not add to your users some warm and fuzzy filling towards you, just the opposite.
If you want to experiment with that - allow user to choose, in clear and easy form, did he wish to distribute in that way.