Piwik supports an opt out iframe, wich may supports your needs. But in some cases it is more handy to realize opt out / opt in via ajax requests and with no iframes. May due to security reasons.
I had such an project and implemented this nice small plugin wich offers you three new API URLs:
You have to use JSONP Requests, as of the AJAX requests needs to manipulate the cookies. In my current project I use jQuery thatfor, but I plan to provite an Angular-Module in the near future. Send me an E-Mail if you are interested.