No. MixedOutcome was introduced during the OASIS process, but is an optional requirement. It wasn't one of the original WS-TX capabilities. It's very similar to the BTP Cohesions type of transaction, but we've not found any call for it; therefore we only support AtomicOutcome. We did support Cohesions in the original HP-WST product, which is based on the same codebase, so putting it into XTS would be straightforward, but at this stage it's not needed for compliance with WS-TX 1.0 and you're the first person to ask for it in over 3 years. If there's more call, we could add it in the future.
I'll correct myself ;-) MixedOutcome came in just before we went to OASIS.