DataPower firmware 6.0.0.x and 6.0.1.y implement draft-03.
Yesterday released 18.104.22.168 firmware implements draft-04.
Find a working draft-04 sample in this developerWorks DataPower forum posting (with 22.214.171.124 firmware):
JSON Schema Validation requires (JSON) input being parsed by DataPower.
That can be easily done by JSON request type.
Side-effect of request type JSON is creation of context __JSONASJSONX and conversion of input as JSONX.
126.96.36.199 service export attached to this posting has request type Non-XML and JSON parsing is done by JSONiq xfrom action doing store:///identity-json.xq.
This avoids JSONx conversion and population of context __JSONASJSONX.