Some fields in the parcel fabric are flagged as non-editable or read-only, and some are not. If you look at the properties of the Z field in the Control attribute table, you can see that Make field read only is checked, and we are not allowed to change this for some reason. I'd certainly want to do what you're trying to do, so I find this an odd restriction. If a parcel fabric field is not flagged as read-only, such as SurveyDate, then you can use Field Calculator.
My idea for a work around... Export the control points to a regular feature class or shapefile. Do the join and field calculation as you were trying to do. Import the control points from this feature class or shapefile and set the import up so the imported control points will be merged with the existing control points, and in the process update the Z using the imported control point data.