No you can't substitute unless you get a CM that's willing to do that for you....but last year we did not. After seeing the Tonga Toast I could understand why....it's like a small loaf of bread LOL. We weren't even able to substitute a fruit bowl for something else....although I don't remember what it was exactly. We were paying OOP too, so it wasn't because we were on the dining plan or anything.
What we did do was get a breakfast meal, then we ordered the Tonga Toast meal as well and we split both

That way we got to taste everything.
We'll have to remember that you can get it at Captain Cook's.....we enjoyed it (I ate around the banana filling though).