Good point! Oil control rings can get gunked up, typically due to infrequent oil changes but sometimes due to carbon blowing through due to the compression rings being bad. An attempt at cleaning them out is not a bad idea.
A compression/leakdown test would tell the tale of the compression rings. You could also find someone with a cylinder scope (a tiny camera on the end of a flexible stick) to look at the cylinder walls. If the car has sufficient power, and you can avoid the oil consumption, and you are happy with your MPGs, I don't know how much more I would "worry" about it. Come to think of it a nice loose engine might be just the thing for hypermiling.
