Although the transaction manager is fully JTS compliant we do not support EJB transaction propagation to other vendors application servers. But we do interoperate with ourselves. I believe the reason for this is historical, when we first bundled the JTS implementation into the app server the GIOP wire encoding of the transaction propagagation context was not fully standardised. But we did need to interoperate with ourselves so we picked a private encoding - in a perfect world we would simply need to change the encoding id to achieve what you want but then we would no longer be able to interoperate with earlier versions of the AS.
But do note that the EJB spec says it is optional: "The EJB architecture does not require the EJB container to support the JTS interfaces" and we satisfy the requirements in the EJB spec for implementations that choose not to support transaction propagation between servers.