Would it be possible for the drawBehind functions to be extended to allow meshes to be drawn with their material instead of solid colours. This would be very useful in First Person Shooter games to prevent the weapon from clipping through walls.

no, the surface would be shadowed (behind the wall) - unrealistic lighting

Many FPS's have an animation where the gun is pointed upwards when too close to a wall. Some in addition to this have it so the gun moves towards the player slightly if the gun is beginning to clip the wall (0-10cm) then anything past that the animation kicks in.