Has anyone worked on using graphml as the standard for storing GoDiagrams in XML format?

This has been on our list of things to do some day.
Certainly simple GraphML documents would be easy to read and write; you could implement this yourself using GoXml.
But please note that the next release of GoDiagram will have a much improved version of GoXml compared to the one that is in beta now, in version 2.2.2.

There are example GraphMLReader and GraphMLWriter and associated transformer classes in the OrgCharter sample in version 2.3.