Well, a google search came up with this:
http://www.saveyourseatlock.com/
Not exactly what you're looking for, but it doesn't mean it won't work for you.
I usually just thread the U-Lock through the helmet strap or carry the helmet with me.
What about one of those thin cables that people use to lock their saddles down like this:
http://www.onguardlock.com/lockviewe...ble&model=5045
Probably work too. Just thread it through your helmet and the U-Lock.