I ended up buying the U-bolt key locks like this:
The U-bolt does catch the little nub on the bottom side of the ventmatic style faucets and the taps can be VERY securely locked. I tried pulling one open and was using enough strength to start to scare myself about breaking something... nothing came out. Not a drop.
My only problem is that my homemade tap-handles are very wide at the base, and they interfere with the locks. Doh!
I need to fix that, but it's not a big deal. I want to get some REAL tap handles anyway.