Our first idea have been laid down in a paper presented at ASM'04.
A complete definition of the AsmM metamodel can be found
A. Gargantini, E. Riccobene, and P. Scandurra. Metamodelling a Formal Method: Applying MDE to Abstract State Machines. Technical Report 97, DTI Dept., University of Milan, November 2006. [link]
Here you find:
The AsmM Abstract Syntax (the metamodel)
From the AsmM we have derived the XMI format and the Java EMF libraries soon available.