Chris,
> I would like to take 36" diameter 3/4" plywood circles and quarter them into pie pieces. <
You can do that, but understand that what really matters is getting enough fiberglass in the corner. Everything else is dressing that may or may not help. In the case of adding wood, that always hurts unless you remove most of it. Your 75% figure is okay, but not as good as 100%

I also suggest you consider thinner, lighter material like 1/8 inch Masonite or plywood. Then at least some of the lowest bass can get through whatever material remains.
--Ethan