It would be nice if 2 different databases needed different code to write out a usertype, that the usertypes be selected via dialect in addition to @Type.

We could write @Type( "foo" ) and the implementation which there could be a PGFoo, H2Foo, MySQLFoo (and probably selected via some reflected dialect specification when loading types)

this is related HHH-9562 (and could be a fix for that (if proposed patch for it doesn't work past pg/h2)