GGC : How to change readonly property of a childtable cell depending on another cell value?

I have created a GGC containing a master table and its related detail table. The master table contains "ApprovalState", "CompletedDate", "Outcome". The detail table contains "Role", "Outcome" and "Notes" to name a few fields. I have made all the cells in the GGC readonly=true by default. I need to now set the readonly property of the Outcome and Notes cells of *certain* child (detail) table cells to false, depending upon the value of the "Role" cell. More specifically, if the "Role" field of a child row is "Underwriter" I need to enable him to enter values in the "Outcome" and "Notes" fields corresponding to his row only. All other rows and fields must be uneditable for him. How can I achieve this?