I think you would have to have a boolean like TopSideCollide: Boolean in the platform object which says:

If (Player is falling) then (engage collision detection on platform object) else (Let him through)

I'm going to assume that the player has x and y velocities, so when the players Y velocity is positive (going down the screen i.e falling) then make the collision detection effective. I think I understood your question correctly but it took me a few passes before I got it. Let me know if this is or isn't helpful.