HI,
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. Another thing is how shall it be accessible from an execution?
In my opinion you could use variables for this purpose. Especially that 4.4 will have support for variable declaration within jpdl so everything stays on the process definition level. Your JSP pages can access process variable and get interesting value for a given outcome.