That rather depends what you consider “the cycling community” to be. I’d very much consider CB to be a part of that - whilst he is an advocate of cycling for transport, he’s certainly also still a sport cyclist. I also note that his mum was out for a sporting type ride with a cycling club when she was killed after being hit by a vehicle (and FWIW I’ve no idea if she was wearing a helmet, but from the reports I’ve seen it wouldn’t have made any difference). I’d also consider the CTC to be a part of the cycling community and they certainly aren’t pro helmet.
I’ve certainly seen it suggested that such risk compensation takes place and it would be surprising if it didn’t. I don’t think there are any proper studies though - it would be even harder to do properly than a normal helmet effectiveness study. I’m sure I’m far from alone in finding that I’m more prepared to try things when wearing protective gear (and that sometimes I wear it simply in order to void wimping out!)
Sorry to hear about your brother. He is one anecdote which demonstrates that a seatbelt doesn’t necessarily save you from head injury. Though he’s far from a unique case - I’m sure I’ve also seen stats on UK deaths in car accidents which show that more motorists die of head injuries every year than cyclists (here seatbelt usage is close to ubiquitous, so almost none of those will have been without a seatbelt).
This is of course also one where we should consider harper’s point about exposure times/distances. On the basis of deaths per passenger miles travelled I’m sure that more cyclists die of head injuries than car occupants. Even on the basis of deaths per time spent doing the activity, so much more time is spent in cars than cycling that it is likely that is still higher for cyclists. However I’m not sure either of those is actually important if debating compulsion (I know we weren’t particularly, but it’s also an interesting point to debate) - surely the only important figure is absolute numbers of deaths and it appears from the statistics that compulsory helmets for car occupants would save more lives than compulsory helmets for cyclists.
edit: just wanted to add that I’m really enjoying this debate - it’s incredibly refreshing to be able to have a debate about helmet use, including a variety of opinions without descending into the usual rubbish