I think it would make more sense to create a "view" object which was a list of columns to display, and grant permissions on that. The view could be either hard coded in the tcl file by the developer, or you could allow user creation of views if you wanted.
I am going to start work on a saved view feature for listbuilder soon and I think you could build on that. It will save all features of the current state of a list, ie: fitlers, sorting, grouping, and which columns are displayed.
I don't think it makes sense to try to grant permissions on the actual columns, but just create a view object that contains a list of 1) diplayed columns and 2) hidden columns that may be choosen to be displayed.
