I am afraid that this is not the right way to go (at least as far as I understood it). If we let custom (unknown and unlimited) attributes to be placed on transition (or any other element) we will not be able to validate the xml against xsd.
-- agree to it completely.
Another thing is how shall it be accessible from an execution?
-- i was thinking from activity as it has api for incoming and outgoing transitions, each of these transitions could have the attributes..but may not be right way to do it.