The ModuleControl is only supposed to be the entry point of a feature, if you need to show different things based on parameters you should implement a supporting page in your feature not try to pass extra params to the CMS page that hosts your modulecontrol. The modulecontrol does not own the url for the cms page. What if every feature on a given page wanted to pass extra parameters to the cms page url, it would be a big mess. The ModuleControl needs to be able to play nice on the page that may contain any number of feature instances that it doesn't know about.
The typical scenario is to have a list of items within your module control with links for each item in the list that link to a supporting page for the details passing in the pageid, moduleid (mid) ,and any extra feature specific params you need in the url for the supporting page. This is how all included features work including the forums, the blog, webstore, etc. Don't try to do everything for a complex feature inside the modulecontrol.
See also videos 22 through 26 of our developer tutorial series.
Keep in mind that the CMS page url is already using url mapping to map the friendly CMS page url to /Default.aspx?pageid=x so it really should not then have more parameters tacked onto the friendly url, also it has one and only one canonical url for seo purposes and that canonical url has no extra parameters. See also How mojoPortal Works.
Hope that helps,
Joe