My neighbour, who, based on past events, should probably invest in a pair of flame-retardant pants, tells me she asked for Ohana Bread Pudding at Kona on the Dining Plan and got it.
Anything is possible. People have reported asking for various off-menu things at 'Ohana before (like things from Kona). I personally do not ask people to bring me things that aren't on the menu, because I think that's out of line. But people do those types of things and I'm sure some are successful and some are not.
If that is your wish, then you should certainly ask...you can always leave more when tipping.
I noticed on our last trip in May, that a neighboring table at Trails End Buffeteria had asked the waitress to bring in strawberry shortcake, from the Hoop Dee Doo next door.