Have you ever run shallow in a well trimmed flatback? I am not blowing any smoke here. On a plane and trimmed out, I can skip across less than a foot- much less. I'd be surprised if some of the spots I crossed were not 8" or less. I would not do it on purpose nor often, but when that flat comes up on you, it's nice have the flexibility. You can routinely run in a foot on a plane.
I can idle in 15" to 18". This is all with a standard jackplate, not a porta-bracket.
All this being said, I cannot imagine that the expense and complications of a tunnel would be worth it.
Not sure where you are, but if you want a test ride in some shallow stuff, I'd be happy to oblige.
