I couldn’t decide how to best avoid oxygen ingress during dry-hopping, so I carefully poured 25g Citra hop pellets through the airlock bung hole before flushing the headspace with CO2 via a plastic tube inserted a little way. It’s probably not 100% effective since doing so only dilutes the oxygen introduced with the hops, but it’s better than nothing.

Going forward I’m hoping to rig something to the inside of the lid which will allow me to insert the hops right at the start of fermentation. I’ve seen several devices which use magnets to keep the hops suspended in a kind of hammock at the top of plastic fermenters, but that’s not going to work with a metal bucket which you can’t see into. At any rate such a device needs to be airtight until deployment time if there’s any truth in the theory that the CO2 produced during fermentation can take away some of the hop oils as it vents out of the vessel.