I don't ever use continuous light in stacking (sadly I'm next to a road with vibrations) which is why I've not chimed in at all re metering/preview/green button in this thread. But I did get my cameras out and try out a manual lens on a bellows to test stopdown metering.
Stevebrot, you've put down exactly my experience. Including the bit about "In M mode, pressing the green button will result in change of shutter speed to match the current set aperture", which I have always thought stop down metering is. (One special little Pentax wrinkle though, on Pentax auto bellows A there's a button you have to press and twist otherwise a Pentax-M lens, even if set stopped down, still remains wide open. Never knew that - because never had a Pentax-M lens mounted on this -A bellows before... By sheer coincidence I'd mounted 50-m macro which has this as an issue, as opposed to something like a Componon which doesn't have this issue and I just and only set whatever f/stop the lens is sharpest at so stack for acuity.)
I've never touched a d810, but my reading of Michael's post is I get the feeling that Nikon has a sort of "realtime liveview" thing going on, ie almost a continuously changing constant preview. So it shows you what a photo would be like at the f/stop, iso and shutter speed settings you have dialed in, and if you change say your 8 second setting to a 1 second setting the liveview display shows a darker image, to reflect what the photo would now look like with 3 stops less light. I maybe wrong - maybe Michael could confirm/deny. I can see the benefit, it'd be nice to have, +-EV is about the nearest thing in the Pentax universe. The only other bodge that might at least speed the process up a little would be to preprogram in a few bunches of bracketing at 1/3EV, make the cam take these and then scroll through the shots magnified. A chore, a bodge, not how Nikon does it, but like many cases in Pentax land it's a case of making the best of the gear so it "almost" does what you want.
Ps Who is Nash