Citizens,
I tried the plug/replug method on repeat for about half an hour to no avail.
Was about to go and buy another (different brand) unit and thought "bugger it, I'm an electronics nerd, I'm gonna fix this even if it's no longer water resistant." Pried off the bottom with the charge connector, disconnected the battery. Left it for 10 minutes. Reassembled. Plugged the charger back in and got the charging light! Battery back to 100%. Glorious. Thanks all.