I've finished designing the HV and display boards. Both are 18mm in diameter and stack one on top of the other, and both are double-sided. When they are connected together, there will be a gap of 5mm between the boards.
I've incorporated reverse-polarity protection for Vin on the HV board - so it won't go up in flames if the battery is inserted the wrong way. The driver chip for the HV includes short-circuit protection too, a nice safety feature for something intended to be worn, but it has also saved the power supply on more than one occasion - it is very easy to short things together when prototyping this stuff.
The display board is unusual - the holes for the nixie tube leads are arranged in a rectangle rather than a circle. This is necessary to fit everything in to the extremely small form factor.
Here are the OSHPark Renders:
So, next up is figuring out a case for the battery and the boards.