With these impedance mismatches between XML and JSON in mind, they really actually settled on the simplest possible mapping between the two types of documents that preserves full fidelity for round trip conversions.Particularly the "really actually" part, which makes me think the author was pleasantly surprised :)
After getting some external support, and clearance from the appropriate groups within IBM, I'm pleased to announce that we've submitted an internet-draft to the IETF, that grants full rights to anyone to implement. You can find it here: http://datatracker.ietf.org/doc/draft-rsalz-jsonx/.
If there are things we can do to encourage its adoption and use, please let me know. We hope you find it useful.