Download: http://www.nexusmods.com/morrowind/mods/43774/? | http://download.fliggerty.com/download-90-1135
Just a simple change. It adds bridges to "in_sewer_canol00.nif" and adds them as the new mesh "in_sewer_canolbridge00.nif". I replaced the objects in the CS using search and replace then went into the cells and removed objects which collided with the bridges.
Since this uses a plugin, it should be fully compatible with mods that add sewers using the same tileset. As a bonus, people can use the mesh as a modders resource or make mods dependent on the plugin.
Enjoy! I hope this makes your exploration more convenient.