I've taken a shine of late to the ATTiny9. It's a microcontroller in a SOT23-6. Like, the size of a grain of Arborio rice. You only get 4 I/O pins (and really only 3 unless you repurpose !RESET), but projects like this only need 2.
So I don't have a need to do so, but I think I will spin a rev of this project to use the tiny9 just to see if it can be done. I hacked up the firmware with "if defined" to differentiate between the tiny85 and tiny9 versions of the code. I haven't checked any of that in yet because it's not tested, but I will do so once I confirm that it all works.
The hardest part is going to be TPI based programming. The ATMelICE can do TPI, but avrdude doesn't support TPI on it (just ISP and PDI). So I am probably going to start with ATPROGRAM on a Windows vm (ugh). If it's possible to hack avrdude to add TPI support for the ICE, I'll do so.