The only problem with handcuffs is so many people have a key. I know I've carried one since I worked in security 30+ years ago. The U shaped lock under the seat on the ST3 can be used also. Locking the helmet through the face shield opening around the chin bar works well.I use handcuffs: one manacle locks the D ring of the helmet and the other locks the grab bar behind the rear seat. You can lock two helmets together by locking one manacle through the D ring of one helmet, slide the cuffs under the grab bar, then lock the second manacle through the D ring of the second helemt. One helmet sits on the rear seat while the other hangs over the rear fender.
+1 I've used this system for years as well. When the wife is along, her jacket sleeve goes inside mine and connects both helmets together. If you do it right, your jacket covers and hides the helmets. Since I've never used the U lock that came with the bike, I should remove it for an easy weight loss.I always keep my side bags attached and carry a cheap bicycle combo lock. Run the cable through the frame. Works well to lock you jacket up like this too. Run the cable through the sleeve, through the helmet and it's good. I wouldn't leave it like that overnight or in a bad neighborhood but walking around a new town, etc... it works.
I use the same Fastbikes cable lock or the flimsy stock cable, whatever is easiest at the time.In my issue of FastBikes I got a freebie helmet lock cable. I'm sure you can find one on ebay or something relatively cheap. Just loop it on the frame, wheel, etc.