Also, shells and editors (i.e. emacs) also obviously have internal development environments, but embedded UML is somewhat less plausible here because it would be proportionally larger than in the other examples.
User environments, such as GNOME and KDE would also be interesting. They could export their UIs, allowing them to be modified on the fly by the user or by scripts. If one UML could be shared by the entire environment, this would allow UML to pass information between them in a standard way using standard tools and would be another way of integrating them cleanly.