The problem with osage and a flat belly is that the earlywood and latewood are two entirely different beasts. If you try to leave it flat, flat then you will likely end up removing too much soft earlywood. Leave it flat almost throughout tillering then round it off some especially near the handle fades.
I personally wouldn't bother heat treating it like a whitewood. I think heat treating is a great way to introduce some fancy shapes and will work very well to correct twist but it won't help with it's compressional properties too much.
So you don't fancy doing a deflex/reflex recurve then??
I don't blame you!
I would also forget the pyramid idea and recurves or at least alter it slightly.
Go for 1 1/2 wide for 10 inches or so out from the fade to midlimb. Then taper into 3/4 wide at the base of the recurves. If you are too narrow near the recurves they may start to twist. Remember bow making is about spotting problems before they happen and you can always remove material once the bow is stable......don't ask how I know this.