Actually, we did research this option, however Arjuna's codebase left an impression of a much more mature implementation (though naming conventions are a bit off, legacy...). E.g. Atomikos' failed to start if there was any unresolved recovery log.
The pros are Atomikos has WS-TX, while Arjuna has it only in JTS version, which is GPL'ed.
Anyway, once I'm positive about all changes in the TX core, it would be a piece of cake to add this support, just make sure to plug into the /core/src/main/java/org/mule/tck/AbstractTxThreadAssociationTestCase.java for integration testing.