I don't think it is possible to build a proper PageObject on top of Mink, because of the goal of the library. Mink simulates a browser. All drivers controlling a real browser don't store the state. The browser itself does. So the page becomes unusable when you change something.

A proper PageObject library would need to store the state in the page, not outside it. This implies storing it in PHP. A good choice to achieve it would to be to use BrowserKit and DomCrawler for this IMO.