are you certain it's coming out the exhaust? if so, my guess is it is also happening in forward when you are at about the same RPM. (the engine has no idea you're in reverse so it will smoke no matter what)

Here is an old tip that seems to still hold true- if it happens under acceleration, it is usually valve stem seals. But, if you go down a hill, left off the throttle and bog the motor a bit and it happens then, it is usually rings.

If it happens under acceleration and first thing in the morning at start-up, you likely have leaky valve stem seals. Also, don't count out that if your valves are way off on adjustment it can happen under acceleration as well.

kt