From what I've seen, it's usually down the front of the shirt if you're shooting prone. Buttoning the top button of your shirt helps alot with hot brass going down front or back. The case deflector works pretty well. If you start having problems with hot brass, get one of the older M16A1 plastic case deflectors. The Army used these on the range. They're a one-piece plastic thingy that simply fits into the space of your charging handle and snaps into the scope mount hole on the carry handle. The deflector part is HUGE. Not that it will get in the way, but it's way bigger than the A2 receiver one. If that thing doesn't protect you as a south-paw, then there's no hope. They're light weight, and you don't even notice it's there. I'm not 100% sure it will fit on the upper with an A2 deflector, but it's just plastic. I'm sure you could just cut a clearance if it doesn't fit out-of-the-box. They're pretty cheap too, when you find them. I saw one for sale recently, but I don't know where it was or how much. Somebody here will probably be able to give you a line one one. If I was left handed, I'd move heaven and Earth to find one just to have. If you're getting a flattop it won't work, as you need the carry handle to mount it. If you're mounting a scope in the scope mount hole, you'll probably need to figure out a different mounting scheme, but that should be no problem.
If worse comes to worse, get one of those brass catchers for the AR and just take the bag off/modify it to suit your needs.
Ross