Huh. That simplifies that massively. Same question, different direction.
How viable is it for dwarves to dig a tunnel, not a channel? One that is underground, not visible from above ground, but that can rapidly be converted into a channel when the top of the tunnel is removed. A "Build what you can in secret and underground" kind of option. They can probably build most of the locks without removing the topsoil. Before the whole thing is revealed for those constructions that need surface access? I've been looking at the maps, and Zulfbar is an endpoint, not on the road of silk. So most trade going by there is with dwarves, and probably, by dwarves or EIC as well.
Basically, use a different kind of subterfuge to hide the digging? Maybe even boost it with Ulgu Illusions for known Marienburg spies, if needed.
You'd still need to convince the dwarves for the need for secrecy, succesfully bluff Marineburgh and deceive their spies, as well as paying higher build costs and causing a lot of grumbling, but is it possible/viable?