I just checked, it would be possible in the current codebase. Actually we do just that on the Elasticsearch side; see {‌{org.hibernate.search.backend.elasticsearch.search.predicate.impl.ElasticsearchSearchPredicateContext}‌}.

The question remains: is this really useful? Performance-wise I would expect the ToParentBlockJoinQuery to already filter by the parent ID, and therefore to not benefit from any additional filter such as the tenant ID…