Part of the problem could be because there exists an actual physical page MyPage.aspx so you should not create a virtual cms page with that url.
For developing custom features I generally avoid the need for query string parameters in the module control because it can live on a page with other features and should not expect to be able to control the url. I mean if there are multiple features on the page and they each need different query string params it gets messy and complicated. Generally in mojoPortal features if I need query string params I only use them on supporting pages of features, never on the cms page itself.
For example consider the forums feature. The forums module is the part that loads on a cms page /forums.aspx on this site and it has a list of forums, it does not use or expect any query string paramters (other than the pageid but this is hidden by the url rewriting) on the cms page, but if you click a forum it goes to a supporting page and passes all the needed query string params in the url to the supporting page. When using supporting pages you should always also pass the pageid and mid (module id) params to keep the context of the cms page and your feature instance.
See also Developer Documentation.
Hope it helps,
Joe