Diana-
I have always used full throttle in a Hammerhead, the throttle position wouldn't be the cause but the fact that you have a float carburator, which relies on gravity for proper function, as you go negative the float will rise and cut of fuel then when positive G's return the fuel comes rushing back in. And the location of the cloud seems consistant with the return of positive G's.
The color is really insignificant in this case because even a very rich mixture will look grey, or white, when disbursed in a cloud of this size. The size of the cloud, and the relatively small amount of oil lost, are good indications this is mixture related. Unless smoked, as in injected into the hot exhaust it would require a very significant amount of oil to create such an obvious cloud of oil, remember that breather oil is misted NOT smoked.
In addition to mixture there is a slight possibility that some of the oil sloshing around the crankcase got into the cylinder, possibly thru the rings at the bottom of piston travel, and got smoked. I have seen occasional puffs of oil smoke in Stearmans from manuvering and must assume it occurs in other aircraft with the exhaust out of view.
Tom-