I have two bikes with the Kryptonite U-locks and holders where the locks have a tongue on the shackle by the lock that slides into the holder. You slide a button on the mount to release it. I wouldn't call it perfect but they do work well. I have the mount about 2/3s of the way up the seattube on the left so the shackle hangs down and forward, ending up just lower than the seattube WB cage.
The weight is centralized and not too high on the bike and the lock is almost always accessible after I have parked the bike on a rack. Also clear of panniers and bags. Not dependent on a rear rack. Lock on and off is easy and doesn't require the key.
Ben