Partial movement?

Just wondering how it'd be possible to do a mini-game type thing in RM2K3 that's similar to say.. the levels in super mario where the screen is constantly moving right and pushes you with it? But vertically, say a ball rolling down a hill, you would only really be able to move the ball left and right, not back up the hill?

Does anyone get my meaning?

Need to figure out how I can only allow the player to move left and right, whilst the movement of a hero "up" happens automatically (So events and things happen normally if hero collides with them, and could hit the end "teleport" to randomise the mini-game a bit)