I think I may have a solution for you. Try this. The OP brings up the same question you did, and I think the guy on the end provides him a solution. How well it works, I'm not sure, but you could try that.
Note: The guy at the end has some pretty horrible formatting skills.
His solution, in short, provides a way to scale based off of a resolution you target, and then make changes for when there is a change. I've seen this done in a few XNA examples, where the developer based his resolution off of 1280x720 (mainly for UI purposes), and things were scaled down for lower-resolutions, albeit that XNA does this inherently and he didn't have to life another finger to make it happen. But that is basically what you're trying to do, right?