Currently it is not possible to copy the value of an attribute to clipboard when the design is locked, since the "Edit Attribute"-window is not available.
It would be handy if there was a way to copy the value of an attribute to clipboard without unlocking the design, e.g. to copy the part number of a component to search for it on the internet.
This is also true when an attribute has been made un-editable in the design using the "No Override" option within a Library Part. Sometimes you want to be able to copy an Attribute where "No Override" has been selected but you don't want people to be able to edit them as they are important part specific attributes for creating BOMs such as Manufacturer Part Name/Farnell order code etc.