Paul T.
Veteran
The big issue isn't parallax, it's that the angle of view of the EVF changes with the focal length of whatever lens you use, while the OVF only has two zoom levels.
Parallax would indeed be the big issue for a focus peaking overlay, as proposed; the view between the sensor, and the VF window, will never completely coincide. The angle of view would also make it trickier, but while you could reduce or increase the overlaid EVF image in size, via software, you couldn't line it up accurately.
Hence focus peaking will only be possible with the EVF.
It seems that legacy lens owners should feel grateful, merely that an adapter is available. Focusing the thing is perhaps a luxury! I doi hope Fuji take notice and add focus peaking, to at least expand the options.