I never use a helmet lock, even though one was on my last several bikes. Keep my helmet with me, they are Damn expensive.
Plus, when I was in college, well one of the colleges I attended, one of my fellow riders had his helmet stolen off his bike. It was locked to the bike, via the helmet lock. I beleive he had a CBR600, but that is a long time ago. Some schmuck, cut the strap on the helmet, so this guy comes out and all he has is the D-ring locked to his bike! He was bumming, it was a fairly new, high $$ Arai or Shoei. This was during the day, in a very visable area, with people walking by all day. I came out, carrying mine (for some reason I never trusted them, plus that way all the chicks knew I rode a motorcycle ;-)) and he was just standing there trying to figure out how to get home, since CA has/had a helmet law at the time. I couldn't help him, as I only had one, and no car at the time. We were tring to work it out when someone offered that they had a truck, ramp and straps (dirtbiker).
I would also guess that the standard type aren't very difficult to beat. Is it that hard to pry them apart enough to get the helmet off?
If you really want one though, there are some aftermarket ones that fit under the seat and secure with a cable, or are a sort of locking bar end you slip the D-Ring over, or one that you install under neath your plate and has a locking hook on each end. Called the "Helmet Gaurdian" I think.
Spike