The nesting connector is identified in the UML2 specification in
either the Superstructure or Infrastructure documents. I don't
have the 1000+ pages with me at the present

, but it is shown
as an alternate representation for package containment - see the
diagrams in the appropriate section. Therefore it would seem to
be eminently suitable for the usage already discussed in this
thread.
I've been recently pondering the difference between containment
and compositional aggregation. Another tool makes the
difference "visible". When you nest (contain) one element
within another, the browser reflects this, but changing the
hierarchy appropriately! If you change the browser, the
relationships are removed/added to the diagram. It would be
great if EA could do the same!
Paolo