Since asking this I have done several dives with the Duo on my helmet and also tested it underwater. Admittedly its only been shallow stuff (Dub Cote, Bagshawe, Home Bank, Frakes, Langstroth sumps, Rowten sumps). I have turned it off and on underwater which probably isn't the best idea as I think the switch would be the weak point, but its been OK. I open it up and let it dry after a dive.
I did take it off for a dive in Capernwray as I was going below 10m. Long term I will upgrade to a Nora or some such.
Someone on another forum did contact me about a custom housing etc, but the costs involved are pretty high. Personally having had a look (as a non-engineer) I don't see what else could be done to make it more waterproof without getting into the realms of cutting more o-ring grooves and the like. The chances of me having the equipment or talent to do that without knackering my light are pretty slim.