I just posted this fix on bikeforums (using my other user name)
Swapped over to Dura Ace 9000 mechanical when building out some new bikes, the problem was that all wheelsets I have only had 10 speed freewheel and the free hubs on them could not be changed to the 11 speed version.
So riding the 10 speed cassette using the 9000 derailleurs was a bit of a comprimise and it did not have perfect indexing due to the difference in the cog gap between 10 and 11 speed cassettes.
To make the 11 speed cassette fit, I removed the smallest cog (11 teeth) and reused the lock ring from the 7900 cassette which fits perfectly into the 12 cog.
The set-up works flawlessly so far, with perfect indexing given the 10 speed cassette now has the smaller 11 speed cog spacing.
This especially advantageous if you have expensive 10 speed wheelsets that can't be upgraded, (e.g Reynolds 2011 RZR's and pre 11 speed Shimano wheelsets) or you don't have the full budget to upgrade.
Hope this works for others in the same position as me.
In this photo you can see the lockring fits perfectly with the 12 tooth cog and allows the cassette to be tightened normally.