I don't think I'd lose the doghouse either. I am planning on building this boat as well starting next month if all goes as planned, and I was hoping for a boat with a bit more shade than it has as drawn as well. My uses would be along the coast of the Carolinas and down in Florida along the ICW, so the shade is a big deal. My plan is to continue the roof lines all the way back to the rear of the boat, much like Doug Hylan did on his 29' Marsh Hen http://www.dhylanboats.com/marsh_hen_plans.html. I am waiting to see what Graham thinks about the idea before I proceed. The biggest issue I see is the low roof height, but a couple access openings on the roof with removable panels can make accessing the boat easier.
While the Marsh Hen is 9' longer than the OB20, the overall look would be similar. I plan to make my front windows hinged in order to get a good breeze going through as Chick suggested.
I think if you are really looking for something to cruise Powell there are better options out there than to hack up the OB20 to suit. It doesn't do the boat justice to stray far from the intended look.
John