I've also thought about trying a sewer game, as myself and one of my other local gamers has the Operation Hades box. Thus combining two tile sets would make a nice game board, just it would be flat instead of 3-D, yet instead of trying to go through Tactics rules since we play Warfare, we'd use Warfare rules.
My thoughts were:
Soldier only armies; and probably no more than 150 AP.
Units would deploy on opposite entrances to the sewer, and could do Unprepared Deployment to represent the narrow sewer entrance through drain openings; as an entire platoon could not fit at same time.
Friendly squads could not pass through each other while in tunnels; or add some form of difficulty to it.
The Zombie factor could be done by as the first squad enters an open area of the sewer system, roll 5 combat dice and for each hit marker, that many zombies "come out of the shadows" with a fast move action to close range. The squad would get an attack reaction, then the zombies would attack.
Would probably place an objective to capture somewhere in the center.
My current thoughts, but not ready to play it. Your idea of permenant soft cover, I'm not certain of that, as most sewers are just cement walls and stinky water in the darkness, so I'm not certain of what is providing the cover. Firing at a corner or a squad behind a pillar would provide some cover.
Thanks for starting this discussion.