General mapping rules from DATEX II UML models to JSON Schema have been defined. These are already supported in the DATEX II schema wizard: https://webtool.datex2.eu/wizard/ , which allows the selection of generation of JSON Schema as an optional alternative to XML Schema.
The DATEX II JSON mapping is yet to go through the standardisation process, and there is a chance that technical details may change in the course of that process. But today DATEX II users can generate JSON schemas using the wizard and use them in specifications for interoperable services.