Very cool project... inspired me to get started on a nixie clock a few days ago. I gotta tell ya', making these things from scratch is a real challenge... After scouring over online schematics I just ordered a ton of MOSFETS, schottkys, inductors, HV caps, 25 MPSA42s, etc. from digikey only to realize that to multiplex tubes I also need HV PNP transistors. :o
I'm also using way too many transistors, right now with my current schematic for only four tubes I'm at all 25 MPSA42s, 4 MPSA92s (anode switching), 14 general purpose switching transistors, and total of 13 IC pins just to switch the tubes (not including the pins of the ATTiny45 controlling the high voltage PSU).
Kudos to you for making a very clean, non-parts-intensive build (albeit with the help of some control ICs :P).