you're probably best-off with tall quads (mdl) with tga textures (or even just overlayed transparency) that are animated (or programmed) to bend down, and you'd make them do this whenever the player (or any character you deem fit) is within a certain distance from them. you wouldn't want to use real collision detection functions due to their slowness (except c_scan might not be so bad).

hope this gives you a good idea

julz