Jordan had the brilliant idea of a valid use case for REP/REQ 0mq support - use it inside a filter.
This would totally solve the need for an exec filter plugin. I will make it happen.
Initial pass merged in master.
Handles either modifying a field OR the entire event. There's a small timeout issue to be resolved. If the reply never comes back, the filter hangs waiting FOREVER.
zeromq filter is in.