One of my Valks came with a Kuryakyn Iso Grip end cap helmet lock. It uses a vending machine like key (added to the bike's key ring). Only one side, but you could do both.


However, a word to the wise. Whatever rig you may choose, ending up with the inside of the helmet hanging up is a great way to get a swimming pool in your helmet, if you are unlucky enough to be away from the bike during a rain storm. (like the two full face above)
Also, it has occurred to me that any asswipe that would steal your helmet might just cut the chinstrap off with a knife (helmet lock or not), since I think most of these thieves would be stealing just to steal, not to actually use your helmet (thousand dollar Shuberth's, Shoei's, Arai's aside).
If I'm in that kind of area (rare), I take my helmet with me.
PS, don't arrange to hang it near the exhaust either.
The guys that crack me up are those who set it down, crown up, on the ground under the bike. Then they never look to see if fire ants moved in during lunch (for the sweet tasting sweat) and just throw it on. I sat on a fire ant nest once in AL. First time down there (USAF), and I never did that again. The locals know all about them. It might even be possible to defeat an indecent exposure case with a fire ant defense.
https://www.youtube.com/watch?v=Rxh9O7zZkok