This is a grand idea! An actual Cargo vessel being escorted through the maps. It'd be lore friendly and pretty fun, and require a lot of tactical sense for the ambushing and counter attacks. Scouting would be extremely important, so Squid's would have that role, while the Galleon would likely be a major escort on the target.
Problem for it: How do the defender's win? Is there a life limit to the Escort's, or is it a time limit? Maybe that would encourage Muse to have the Game Timer included in the overlay.
Also, I like that idea of having the spawn point move with the Cargo vessel. It makes sense, but only if your team "owned" the vessel, or that you had captured it. Overwatch style, as it were, where the enemy can slowly push the Cargo backwards if your team doesn't control it for long enough. Spawn points would vary depending on where you were on the route, maybe three stages of spawns, each progressively closer to the "Goal" without them being directly on the cargo.
Might be able to finagle some more maps out of them with that kind of mode.