Solved it! Well sorta, it's basically doing what I already suggested, and using a bit of creativity, and some of what deadeye was onto as well.
I should point out that this will only work "properly" if your game is flip screen like the two I saw on your website, with the black guys. Course if your game scrolls then this whole aspect ratio thing is a farse because you so wouldn't need to worry about it. But even then, it would still work!
Ok this is what you do:
Make your layout the size of the largest fullscreen display currently available, don't worry this wont effect filesize or anything like that.
Create your game >>>>>>> AT THE CENTER OF THE LAYOUT <<<<<<< This is very important, it'll not work right if you don't do that. continue to make your game in 320x240.
Now, around your actual game window within a window. Create a border, a nice pretty graphical border, how it looks depends on your game. Put this border on the OUTSIDE of the 320x240 window. Because you never want your 4:3 users to see this border, it's not for them, it's too good for them and they're not allowed to see it.
Now then, create a tiled background, on the lowest layer. Make sure the tiled artwork matches your border you just made, so it looks like it's part of it. Cover the entire background with that tiled background image.
Right, by now you're looking at it and probably thinking "Nobody is going to play a tiny window like that on a huge resolution you idiot!!!" But that's cool, that's exactly what I want you to think, and if you're thinking that, then you've done this right so far.
What you need to do then, is create a layer which will sit above EVERY OTHER LAYER, it wont scroll it wont move, and it has to sit exactly over your 320x240 playing area, taking into account the borders too (hey maybe the 4:3 crowd will get to see those amazingly rendered borders afterall!) Add a pixel shader to it, magnify, or lens, up to you whichever works best.
Now the magic
By using sysinfo, you can get the height information, which you will then use to change the size of this magnifying layer, depending on whatever resolution the user is running in. You'll have to use some math to get the width correct, but I'm sure someone else will be able to help on that score.
You may have to tweak the size of your magnifying lens somewhat, to get it working right, it may not be as straight forward as just giving it a height to use.
What you'll end up with, is a playing area which is always as high as the users resolution, and if you did it right, two nice borders down either side, and a tiled background to fill in the extra space, which using this method, wouldn't be much wasted space really, unless they've got some weird super wide monitor or something.
As for the 4:3 users. Well it all depends really how the magnifying layer behaves on those displays, so it may just be a case of putting things in groups and turning off the magnifying layer and turning on a regular window set size group, depending on whatever resolution they're running at. And I'm pretty sure that's entirely possible too.
I dunno, tested it and it seemed to work ok to me. So give it a go, see how it works for you.