Actually looking into the code I see we have a solution you can use by adding this to your user.config file and then touch web.config to make it reload settings:
<add key="UseRawUrlForCmsPageLoginRedirects" value="true" />
But my advice is don't build custom features that link back to the the CMS page friendly url with extra parameters, that is working against the grain of the system for the reasons previously mentioned and not what we do in any features we ship, link to supporting pages instead is what I recommend and what I do in all the features I create that need extra parameters for anything. Using this web.config setting is a workaround and using extra params with the friendly url of CMS pages may have other unintended side effects, so it is a your mileage may vary kind of thing if you use that.
Hope that helps,
Joe