Hans,
The ebMS work pioneered this with SOAP well before WS-Reliability was conceived.
Interestingly the code for building this functionality around your messaging service is available as open source - from H2O project.
Further more - the H2O implementation is built around an open SOAP framework and web service calls - so - this means you can implement an EDXL-DE adaptor for H2O - since H2O itself is implemented this way for both AS/2 and ebMS today.
Leveraging all that code would make the writing of the EDXL-DE with reliable delivery a minimal effort - because all the functionality you desire is already built-in to H2O.
Adding that EDXL-DE adaptor would also give big impetus to adoption worldwide since H2O has an existing large global community.
BTW - H2O also supports email delivery methods - which could have interesting possibilities for certain EDXL-DE applications.
Thanks, DW