The problem for me is that I am using a command line interface to version control. I am using the x64 version of SlickEdit, and our version control system doesn't yet have a 64-bit version of the SCC DLL. If you could just distinguish the read-only files somehow, perhaps with a slight variation on the icon, that would work quite nicely.
It would actually, useful, for files in general, even those in the Open tool window, that are not part of a project or a version control system.