Seas =)
The best way to use a custom bridge is, to take a normal bridge and change everything you want (model changing and so on...)
Make sure that "pathing - Is walkable = true".
Make sure you got the correct Pathing Texture.
Make sure that the bridge is close enough to the terrain, cliff (what ever) that the units can walk over it, and don't stop because the bridge is to far away.
( I don't know the claw bridge of UTM - but I think this should help )