What I meant with my question is whether there is a way for Mendix transactions (that it probably uses internally in order to update its data, including DB changes) to propagate to external systems – like if it were using JTA or some other XA/distributed transaction specification. Since Mendix supports those “Java calls” in its flows, thus opening the door to interoperating with other external systems, I was thinking it would be quite a useful feature to be able to obtain a handle to the Mendix transaction and propagate it through that “door” too.

The same question goes for the opposite direction: that is, whether some work done in Mendix (as a result of invoking a web service or any other way Mendix can be called from external components) can be subscribed to an existing distributed transaction, coordinated by an external entity.

Just discussed it with Achiel and Michel and the quick answer is: "no". of course it is possible to create something like this but as we say here: "you shouldn't want to do this". It creates a tremendous complexity and bugs.