Note to self: 'Nards has 3/4" birch B/C sanded plywood for $22 a sheet. That's cheaper than pine C/D.
I ended up going with 3/4" T&G OSB since it was $16 a sheet. It won't withstand direct exposure to weather, but it will in fact be covered by a sheet of EPDM rubber, otherwise known as a roof condom. I'll still seal it first; I have considerable knowledge of the construction of OSB, and I can use materials that will bond with the existing bonder in the wood. The edges are particularly prone to absorption.
Tonight, I'm gonna sleep. Tomorrow is another day, and there will be the working.