I just realised that the 11t and 12t are seperate cogs (was hard to tell so had to go to youtube videos to work it out). So could easily leave them out and just machine a different plug end thingo (the Kappius version of a lock ring) to hold the cassette on the axel against the 13t cog.
Shimano make quite a range of cogs from 17 right through to 28 for dura ace (and also up to 30t with ultegra 6700), which are pinned onto spiders. These could easily be unpinned, and then repinned onto a spider that could either be machined in to the Kappius freehub dome, or to a seperate spider that is then pinned to the highest cog on the 23 of the powerdome cassette, then the seperate spider gets pinned to the Kappius dome (probably the easier option).
Kappius, if you are listening and keen to give this a shot, I'm an impoverished postgrad science student and can't really afford new bike stuff right now, but I'll happily shell out the biscuits ($$$) for the parts (hub, red 11-23 cassette, 6700 or 7900 cassette or at least the last spider with the cogs needed) for you to give this a shot, then test it for you