Apparently there's a piwik plugin that should support A/B tests, but it hasn't been updated in 2 years, and I guess it'd only support A/B tests and not multivariate tests:
While analytics is great to learn about what users are doing on a website, experiments are a great way to learn how to improve that behaviour. It means website owners can make their websites more efficient, and get better results. Every day you're NOT running an experiment is a day wasted learning.
There's an open source tool called Genetify that could perhaps have been integrated, but it doesn't seem to have been updated for a year, and the demo didn't work 100% in my Firefox browser.