I tried the named callable query approach...
<sql-something name="..." callable="true">
{ ? = call schema.procedure(?,?) }
</sql-something>
....but what I get is either a message about native calls not being
supported or some kind of type mismatch exception. (Like stored
procedure has been executed but returned Type=4 when Type=1111 was
epxected, or vice versa.

So this is what I'm using currently:
Boolean result = (Boolean) session.createSQLQuery("select
schema.procedure(aram1, aram2)").setParameter("param1",
param1).setParameter("param2", param2).uniqueResult();

It works but I would feel better using "a proper" solution.

I have read many posts related to this but can't find a satisfying
solution. Most of them are out-of-date, using deprecated constructs.

Best regards
Jarda Z.