I think we should have Etc->Clear Quest Filepath grey out after clicking "Yes" on the Clear Quest Path msgbox, because there is no point in clearing it twice.
Grey out multi-Clear Quest Filepath
The msgbox is created by onClearQuestFilepath() in zq_misc.cpp
etc_menu is a static NewMenu declared in zquest.cpp
The actual clearing is done by ZQ_ClearQuestPath() in zquest.cpp
filepath is a char* pointing to an empty string after a call to it completes.
Here is an example of how to grey out a menu option:
edit_menu.disable_uid(MENUID_EDIT_UNDO, !Map.CanUndo());
edit_menu.disable_uid(MENUID_EDIT_REDO, !Map.CanRedo());