Matomo (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:
your.piwik/index.php?module=API&method=AjaxOptOut.isTrackedYou will get an Response whether the current user get's tracked or not.
your.piwik/index.php?module=API&method=AjaxOptOut.doIgnoreMatomo will set the ignore cookie for the current user.
your.piwik/index.php?module=API&method=AjaxOptOut.doTrackMatomo will remove the ignore cookie for the current user.
You have to use JSONP Requests, as of the AJAX requests needs to manipulate the cookies.
This plugin is compatible with Matomo 3 (see requirements below for more details).