To implement inbound bridge we use subordinate transactions, which are not in JTA or XA specs. Thus we have to use our internal SubordinationManager. RTS bridge is based on TXBridge architecture, which is described in here: Narayana Product Documentation
Also, tomjenkinson pointed me to this JIRA, which might be of interest for you: [KARAF-3749] Add support for the Narayana Transaction Manager - ASF JIRA
So using the Narayana transaction manager within karaf is going to be a requirement.
I'm afraid so