speaking off the top of my head without doing much thinking (probably not a good idea
) I'd say you might look into system that uses an invisible layer on which all objects have a one-color texture. Then you render this layer to an image on which you use the color information to construct the 2d reality...
So, say the background is rendered as black, while all objects that the player can stand on are rendered in white. Then you would have a white "landscape" on black and could (somehow^^) construct your platform world from that...
There was a thread on here at some point in which the author (it was a tutorial) explained how to use this kind of hidden layer-technique with colored areas to make a button layout for a menu. Essentially the program checked: Is the mouse over a red are? yes -> click = start game. green area -> click = quit game... etc.
Instead of using this color code for buttons, you could create the 2d "platformer" world and check 2d collision of your character...
I hope this even made some sense^^ tired
