Michael Musgrove commented:

"In the longer term (possibly in release 5.x which is at least a year
away) we will need to support multiple transaction managers in the same
VM..."

Task is to find out more about this and see if there is anything that can be done in 7.0 to ensure that when this comes in there is no need for an incompatible change in the management model for the tx subsystem. For example, perhaps add a level under the subsystem: /subsystem=transactions/transaction-manager=default/...