Nice theory about the Gas being too rich, and of course you can not check this because it is burning far too much oil. My machinist thought the same thing, and we went through three sets of rings, after going through countless pistons, rings, more rings and more pistons... for 20 years... Even made torque plates, and used special ring break in lube from Hastings. The best results through all this trouble was Total Seal rings, with Hastings three piece oil rings, that Total Seals no longer sell in sets. So, I bought two sets, one from Hastings and one from Total Seal, and it still burned oil, but not as much.... Maybe 150 miles to the quart. Better than the rings that come with the pistons, if you get the correct ones set up for K model pistons, and not the ones that Eastern set up for 45ci rings. They may work if your engine is set up correctly not to over oil, but if not you will burn about 29 miles to the quart.
I know you said, "We've mooted the oil restrictor idea that got kicked around in the K forum a while back but we aint buying that they left the factory like this... " and you are correct! They did change the pinion shafts from the K models to the Sportster models as a running change in 1957. The pumps volume changes little, but there was more oil going up stairs to oil the top, so a bigger hole was likely needed to get oil to the crank shaft. Unless your motor has an original shaft, then likely the pinion shaft was replaced with a later model Sportster shaft. Bigger hole, more oil, soaking down the pistons, rings, and cylinder walls with way too much oil.
Here is an easy way to check though. I welded up my drain plug as I felt it was a nuisance always leaking, but if yours is still working, un screw it with a pan under in and measure the amount of oil that comes out. It should be less than three onuses or so, or just enough to touch the bottom of the fly wheels. If you get a half a cup or more, well then there is way too much!
You will have put new rings in even after you restrict the pinion shaft, but this is the way to get the bloody things to seal. I would have to go back to "Wet up top" to get all the figures. I think it was .090
in the left hand screw. It was .410 before... But I would have to look it up.
I know that this did, work though, and my bike, with the regulard hastings rings, with three piece oil ring, is getting 300 to 400 miles to the quart, and that is running hard. The oil does not turn black in 4o miles like it used to, and my plugs look pretty normal, thogh I think it is still getting more oil than nessisary pat the rings onto the back cylinder. I am pretty happy with the whole set up! I just don't have the time to ride much, or the weather this time of year in Seattle.
Best of luck!