You might like to look at CDIF, which is concerned with interchanging
diagrams such as entity-relationship models where the topology is more
important than the physical layout, and where the physical layout may be
defined with different degrees of precision. See www.cdif.org
I last looked at the detail about 5 years ago, I'm sure it's moved on since
then. They are working on an XML encoding of their existing data model
(replacing an ASN.1 encoding, I believe)
Mike Kay