I used Arduino programming on a STM32 board. You can add any bare C or even asm trickery alongside for things that are not directly supported by Arduino (interrupts, sleep and any other mcu config stuff), while still having Arduino do the bulk work clean and easy.

So I actually used an interrupt for the sensor, but no sleep modes for now except powering off the LCD.
Even after optimizing it for low power consumption I don't expect it to work off a CR2032 anyway because the LCD (or rather the time spent sending data to it +backlight) would draw much more than the cheap odometers. I think a rechargeable 1000mAh LiPo cell would be best.


BTW, I've been thinking what if Nokia made available the tools to write firmware on the 1100.
So you could have the whole rugged phone case with everything that's needed, and possibly using one of the built in connectors to attach the sensor without any hardware hacking.
Or any other cheap monochrome LCD phone..