I've also considered the screw option as Rollon Dave mentioned, but using 2 as Arch stated to shop them being unscrewed (
seen here).
I wouldn't worry about wieght when it comes to bike security, my lock is by far the heaviest accessory I have and haxing a couple of screw scewer thingies isn't going to add much. Plus when you're bikes been nicked, all you gear is going to feel pretty heavy all of a sudden!
I'd also try to find a big screw which has a welded loop, unlike the bent version shown in the 'seen here' link above. If there's no alternative, it's the best option i think.
edit to account for Snorri's points...
digging out would be hard as there'll be a bike in the way.
Explosives would be needed to get them in on many occasions.