Besides Apache, what might make a good virtual device to embed UML into? There are a number of services and applications which have internal development environments. These environments can also be viewed as the equivalents of home-brew OSes, so replacing them with UML might make sense. mySQL uses perl as an extension language, mozilla has both Javascript and Java support, and Gimp has bindings for a number of languages.
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.