If placing civilians is that simple and won't consume too much (computer) resources the I may just add them in then, on all relevant maps.
You'll just have to put them on the civilian team and make them indestructible (or perhaps not if you'd like to be able to kill them..?).
You also might want to script a few of them to walk around, so the units aren't so static. If you script them to follow a waypoint path loop, then they'll just keep walking around for the entire match. And if you make the paths fairly long and complicated in WB, then it will be less noticeable that they're looped in-game