A possible workaround for the issue is to use a custom customizer and the following code:
public class CCCMyCustomizer: StateLessEntityLogic
public CCCMyCustomizer ()
CanEdit ( "extensionAttribute3")
.From ( "extensionAttribute2")
.As ((string ext2) => ext2 == "A");
This must then be delivered via DialogFileRevision, made known in the dialog and assigned to the table.
There is no fix for 7.x or 8.0.x as it would result in a code breaking change. A fix is available in 8.1 in the way of two new columns, DialogColumn.CanEditScript and DialogColumn.CanSeeScript which can now be edited in Designer.