For some reason I keep thinking you're supposed to right-click....something...onto a duct to make it dense or whatever. Like, right-click lead ingot or something.
I won't be surprised if its the duct itself you tweak rather than the servos. The routing seems to pursue a sort of weak A* algorithm (pathfinding) where the "dense" operator just changes the cost of a node. If that's the case, you could make any section dense.
Try crafting the duct together with pulverized lead - that might be an old recipe, though.