I suppose Zoom and Crop/Pan could be locked, but cannot be at this time.
I'd have to look into the code to see how to do these.

Zoom would be pretty easy, as I recall the way it's implemented, and it's a straightforward thing with only 4 discrete choices (1, 2, 3, 4).

Crop and Pan would be more complicated, as they would require dealing with the possibility that the Underlay datasets in two viewers would not be on the same grids (voxel dimensions and/or voxel counts along each edge), as the Underlay dataset defines the 3D "box" from which the image viewers extract slices. I suppose the easiest way to deal with this particular difficulty would be to outlaw such locking. Then it would probably not be too difficult. But of course, then Paul will also ask for the ability to chauffeur such capabilities, and then I'd have to take away his espresso machine (AKA the death penalty).