If I've no extra frames of brood to add to a trap/new hive I throw a queen excluder on there (top-entrance only). I've witnessed that preventing a re-swarm... a lot of the bees all boiling out as if they were going to swarm out only to find that the queen wouldn't be joining them, so they returned. You're an experienced bloke so I imagine you've already considered the queen-excluder preventive measure but thought I'd share, just in case. It's also prerequisite that you actually have the queen (non-virgin) in the box prior, of course.
Cheers,
Dane