I was delighted to be able to use my old (fast) Canon FD lenses on the OM-D E-M1. Mostly the 50/1.4 and 135/2, but not necessarily wide open.
Stopped-down metering is definitely feasible until f/4 or f/5.6, even with this "old" EVF. When the light becomes dim, you won't normally shoot at f/8 anway. I take most night shots at f/2.8-f/4, with the DSLR or the m43 camera.
The Nikon Novoflex adapter in addition has a ring for forcing the diaph to open, so a quick change from wide open to pre-set aperture is possible. The lens + adapter then works a bit like the old 28/3.5 or 35/2.8 PC lenses.
The nice thing is, with increasing stopping down, the DOF will also increase the tolerance for misfocusing. The noisier EVF is ugly but still usable under such conditions.
In addition, most focus shift issues arise typically around f/2.8-f/4, where increasing DOF might not compensate for the shift. Here too, I expect some greater ease of use in relation with mirrorless.
So I expect an even higher level of satisfaction using AI(S) lenses with the future Nikon Z cameras, all the more so because there will be no crop factor. Not perfect, but certainly sufficient for my "slow" photography habits. For fast shooters, there will be the Z line.