I'm guessing valve guides or valve stem seals. Under normal driving or when lugging, oil is dripping past but you're not revving high enough to notice it burning off. At heavy throttle or wot, you're finally able to see the accumulated oil burning off.
My 91 has eaten oil since I've owned it, but a couple deer seasons ago I started noticing a puff of smoke after putting around dirt roads and getting back on the highway, eventually getting bad enough that I'd see it if I were putting up a dirt road and stabbed the throttle a little bit for a small hill.
