If they want Json support for JSR-353 types, one is already published.
https://mvnrepository.com/artifact/com.mopano/hibernate-json-contributor
If they want it for a non-standard API, they are free to use it's source as
a base. License permits it.
They will need 5.2.10 if they need to bind a JsonArray as a native query
parameter.
I've tried making a module that uses PgObject, but that runs into very
nasty classloader problems in Apache Tomcat. A work-around should be
possible with reflection, but it isn't worth it for things where
[sg]etString works.