We're definitely not looking at tying this to a specific ESB or SOI. John and I had some discussions around this at JavaOne last week, so expect some update here asap. STP may well be an option though.
On looking into the STP IM in more detail, it looks like it is more intended for interchange between the various STP editors, rather than acting as a generic format for holding information across editors, which is what I had hoped it might be.
However it still might be useful as an interchange format from the Overlord modeller, to enable the model to be represented in a BPMN or SCA view.
I think John, you and I have all mentioned EMF before, but that's definitely something we need to consider. In general I like the goals behind STP, but it's had a long, slow life so far.