DC Connector for medium voltage PSU output
It's been a week, and I've solved my own problem. My mistake was that I was discounting "Industrial and Automation" applications in my searches on distributor websites (RS, Farnell, etc) and my searches for "DC Power Connector" were swamped with little 12v power jacks.

Once you click "Industrial", all the 200V-600V AC/DC connectors magically appear.

My problem has been solved with Bulgin Buccaneer circular connectors. I'm using their 4000 series connector, which takes a small (19mm) panel hole, and is locking, waterproof, beautiful, and of course, reassuringly expensive.