I've just had a pile of boards manufactured with QFN ATTiny45s and with the XC9140 boost converter, (they're not yet arrived, though), but also ordered some prototype boards for an ATTiny44A variant.

After a little bit of hacking about, it works just fine. It takes maybe 10% more power, but that's really a difference between ~18 µA and ~22 µA - not really outside the margin of error anyway.

The biggest difference is in the calibration code. The feature pin mapping is (as you'd expect) quite different on the 44 versus the 45, so the pin I was using before changed from being OC0A to OC1A. So the calibration code for the Tiny44 has to use Timer 1 instead of Timer 0. That's not really significant, since the calibration code doesn't run from a battery (or even installed in a movement).

The only changes were in calibrate.c and base.c, and all of it got handled by using the AVR processor type macros, so as long as you set the mmcu directive on the compiler properly, you'll get functional code.

Meanwhile, I've ordered some XC9142 chips, which appear to be an updated version of the XC9140. They at least appear to be somewhat more plentiful on DigiKey, so they may be the way to go. I'm going to try swapping some of them out on my test boards to see what the impact is on power consumption.