I don't really have a reason to do this, but I've decided to try to rev this to use the ATTiny9. I patched the source code to use the chip select macros to pick between ATTiny4x and ATTiny9/10. The code winds up being 669 bytes long (won't fit in a Tiny4, alas) and since I don't need an ADC, the Tiny9 is nominated.

Since the project only requires 3 pins, it's an ideal part. I can leave !RESET alone and therefore won't need to play any funky tricks to be able to use it as a GPIO pin except for programming.

The only problem is that I have an AtmelICE programmer that can program with TPI... but avrdude doesn't support using it for TPI, unfortunately.

So as soon as I can figure out how to either fix avrdude or get another programmer I'll get some boards made and try it out.