Some of Fundraising spoke with Darwish Gani and Aaron Landman from Optimizely today, and learned that they offer an enterprise mode which might be legal and appropriate for MediaWiki UI experimentation. Their product is a visual editor that generates jQuery to manipulate the interface (or you can use custom js), then delivers variations to viewers and tracks the results.
Alternative to T76918: Investigate using Facebook PlanOUT for A/B Testing Framework