I have asked our XTS experts to take a look at this for you. To clarify are you looking to have an external application (none servlet) lookup the transaction manager, and integrate with various WS-AT services. At the WS-AT services are you intending to bridge to JTA in order to enlist various databases, then have the client commit the transaction to propagate to all the WS-AT services and the databases that are XA compliant?
In the meantime, there are some quickstarts to get you going but they don't use a client that is not in a container: quickstart/XTS at master · jbosstm/quickstart · GitHub
Within that part of the quickstart tree you will see wsat-jta bridging going on to get databases enlisted.
Hope it helps,
running XTS outside of WildFly/EAP is not something we have tried before. Although it is theoretically possible, it is not going to be straightforward. You will have to make sure that all the required WS-AT stack is registered as well as TXBridge handlers are set up. Also, TXBridge is dependent on our JTA implementation, thus it is important you have it in place too.
I would highly recommend you to move your application to our application server, if that is possible. That would make your job much much easier.
However, if you would decide to move on and try to set up XTS outside of WildFly/EAP, I might be able to provide you with some assistance. But, I would need more information about the environment in which your application is running.