Heres how I would do it: have a Bitcoin Plus (or whatever) widget box clearly visible on the website. Have it tell the user exactly what it is doing, including the current speed. Highlight the fact that it is an alternative to advertisement and that it only uses idle processing time (or if its used together with advertisement, state that its used to help pay for the site). Finally, provide a stop/pause button so that if it really annoys somebody or actually is causing cpu spikes, its easy to turn off.