If UNION support makes it into HQL grammars, it will happen as part of the work being done under https://github.com/hibernate/hibernate-semantic-query which has been discussed alot on the hibernate-dev mailing list. If anyone is interested in helping with that effort it will help get things like this implemented sooner. If not, it gets done as I have time between fixing bugs and other work.

In the meantime, per HHH-6672 the incomplete support for UNION in the current grammars will be removed in 5.0.4. Its incomplete and causes problems in recognizing certain valid queries.