To be clear what I am talking about is properly handling the following 2 situations:

StoredProcedureQuery spq = ...;
spq.setHint( "hibernate.proc.param_null_passing.firstArg", "true" );
...
spq.registerStoredProcedureParameter( "firstArg", ... );

and

StoredProcedureQuery spq = ...;
spq.registerStoredProcedureParameter( "firstArg", ... );
...
spq.setHint( "hibernate.proc.param_null_passing.firstArg", "true" );

The first case illustrates that every time a parameter is registered, we'd have to iterate over hints to see if there is a hint for it. The second case illustrates the reverse: every time a hint is added if it starts with hibernate.proc.param_null_passing we'd have to see if there is already a parameter registered for it.

Partially this is a result of the decision to model this boolean on the parameter registration object itself. But that is, imo, the most natural modeling for that.