I would do it with an animated model......lol....but thats me. I don't like spending a whole bunch of time scripting so an animated model is all I would do.
I would reccomend an animated model for the time being. This is due to their currently being a bug with A6.30 and "ph_poly". While you can use "ph_box", I doubt its the precise collision detection your looking for if a player cross's this drawbridge.