Sure, we're on the edge of spec here, I'm no authority on this one and I'd be curious how that section 4.6.17.3 in specification was meant. Sure, providers have to use some measures (dialects) to support various databases. But I never registered any functions with EclipseLink. Actually, I just created one in H2:

CREATE ALIAS XXX AS $$
String xxx() {
    return java.util.UUID.randomUUID().toString();
}
$$;

And immediatelly used it:

select FUNCTION('xxx') from Dual dual

And it produced random UUID as expected, returned object was of type String. So either I was extremely lucky, or the fact that the registration is absolute must is not true.

In any case, Hibernate's current syntax does not seem to support functions without parameters (as mentioned by Steve Ebersole).

Just to be sure, Christian, your comment is an official stand of Hibernate on this? You tried other providers and they all have this issue? Is my example with H2 and EclipseLink somehow wrong?