it's this flag:

--experimental-query-plan check                                                   comma-separated list of operations to route through the experimental query plan engine; valid values are check, `lr` (LookupResources), and `ls` (LookupSubjects)

so you'll want to give a list of endpoints that should use it. i'm also not 100% sure that we've wired it up to bulkcheck yet, but if it is wired in, it'd be with

check