So for those who might be interested, I've come to some conclusions since my first post. I'll share them here, since they might help others.
Since that post, I've closely monitored the average and SD of the case lengths I was producing, while varying my lubrication strategy. In fact, I ended up coming with a granite surface plate and a dial indicator for more precision measurements. (Incidentally I got significantly more consistent results using the Sinclair steel taper-cut inserts compared to the Hornady aluminum shoulder inserts).
The long and short is that I was WAY under-lubricating the cases. I was using 2-4 pumps of a Franklin Arsenal lube spray (which I think is very similar to the Dillon spray, but I'm not sure if the sprayer is the same, ie has the same amount of fluid per spray) per 100 cases, and then tumbling them in a carboard box or plastic bag.
My new process has me putting the cases into loading trays (50 pcs per tray), and I spray 8 pumps per tray. The 8 pumps are equally spaced around the tray, and at a slight angle downward to ensure that plenty gets into the case necks. I spray each tray just I start sizing the tray before it, so each tray has time to dry equal to the time it takes to size 50 cases.
This has improved consistency from tray to tray, reduced the SD of the case lengths, and done a better job of lubricating the inside of the neck for the Sinclair mandrel operation.
I did try other lubes, but found the lube I already had was doing as good or better job as anything I tried. I did try the wax as several suggested, and it worked quite well, but I just found it very slow and messy.
The only downside to the method I outlined is that you pretty much need to clean the trays after you use them like this. I have about 20 trays for .308 bolt face calibers, and when they get sprayed, they get thrown into a "dirty" box and when I run low, I throw them all in the dishwasher (low heat).
FWIW, after sizing (and trimming / chamfering / deburring) I do clean the brass again, in a wet tumbler (SS pins), for just 3 minutes, followed by a very thorough cold-water rinse. I put the cases in cookie sheets lined with towels, roll a towel lightly on top of them until they appear dry, then remove the damp towels and let them air dry for 24+ hours. Before I prime each one, I inspect it for a clear flash hole.