Currently, it is not possible to attach your own implementation of "Query Cache" and "QueryCachingPolicy".
I think this should be in the backend parameters, e.g. "... <backendname> .cache.query_cache_provider" and "... <backendname> .cache.query_caching_policy_provider", as implementatins "QueryCacheProvider" and "QueryCachingPolicyProvider". If not defined, load via java load factory service with the java "QueryCacheProvider" and "QueryCachingPolicyProvider". And the implementation of these factories shifts it to the original hibernate mechanism in "hibernate-search-mapper-orm" which downloads implementations from "StrategyRegistration".
Ultimately, if no option is defined, then default parameters from "IndexSearcher" will be used as usual.
This would enable building query caching policies and query caches using standard proven mechanisms from hibernate-orm.