Create migration stubs of entities for a multiple value reference field?
Yes. This use scenario is covered by migration_lookup plugin with its stub_id property and making sure it's referenced migration (in your case, a user migration sourced from the JSON array) has defined all the required fields via the default_value plugin such that migration_lookup can rely on it to stub out a user before the stub_id migration can be run on its own to fill in fields for each referenced entity.