I've only just seen this thread but think I may be able to help a little here. I've built something over 25 twincams over the years, both for road and race so I have a little experience of these engines. The OP is quite correct, there are no valve stem seals as standard which is why a small puff of smoke is not unusual when blipping the engine, a continual stream of smoke is another matter. Correct valve to guide clearance is essential but easily measured and either confirmed or eliminated as the cause. Similarly if the piston rings are at fault then there will be lots of oil present in the combustion chambers of the affected cylinders only. In this case I would be concerned about the fit of the valve guides into the head as the cambers all sound to be opretty equal interms of oil and the original pictures show an oil track down the outside of the exhaust guide where it is visible inside the port. There is no obvious sign of oil on the valve stem, so I think the oil is coming down between the guide and the head. This is quite common when inexperienced builders just hammer the old guides out and smash new ones in without heating the head first. The good news is you can buy oversized valve guides which can recover the situation pretty well. If the bore the guide fits into is horribly scored (quite common if carbon has been dragged through by the old guide) then a little "bearing fit" anaerobic sealant will work wonders. I'm afraid all the well meant speculation about camshaft followers, oil gallery location and windage trays are distractions from the primary issue here.
As an aside I wouldn't be too worried about the black sludge on top of the pistons if all the engine has done since it was built iis idled and run at light load and not really gotten up to temperature, most of that gunk is just carbonised fuel deposits that have condensed out onto the relatively cold piston crowns, especially if she is running rich at this stage.
I hope this helps, Nick
