The answer is that it'll fit some, but not others, and even those it does fit it'll be very tight - hence not something you'd want to be doing with cold hands.
Unsurprisingly it fits Petzl helmets such as the Meteor and the Sirocco. I also tried it out on one of Black Diamond's new Capitan helmets, which are coming out next year, and it just fit a size one, but there was no way it was fitting on a size two.
As such, it probably won't be the first choice for climbers, although the irony is that once it is on its low profile nature means you're far less likely to bash it.
Thanks for that. I had wondered if there was a clever 'unslinging' option for the cradle to leave a cord-only attachment for a helmet. My winter lid is a Petzl Boreo, though TBH I most often use a headtorch for dark o'clock starts and late descents rather than when climbing. Round the neck might be less comfortable with this setup?
Sadly I don't have a definite answer, but if it's size two I'd imagine it'd be touch and go. That said, I'll try and head over to Outside later this week to see if they happen to have one in stock, after which I should hopefully be able to pass comment either way.