Very interesting reading, thanks for the link.
Re.the possibility of air getting trapped in the tunnel, just a thought:
By steering hard over and blasting the main prop forward, the water pressure on one side of the tunnel (port/stbd side, depending on whether steered to port/stbd respectively) should increase significantly, and I'd expect that to be more than enough to bleed any air trapped inside.
Of course, it's hard to tell for sure how effective such trick can be.
But it might be worth a try, because if it works, upon arrival after cruising it would be sufficient to routinely give a short blast forward while steered hard over, just in case, before maneuvering.
As I said, just a thought...