Thank you for your reply.
Do you recommend the clean-shutdown daemon even with the "onoffshim" one-liner installed?
With the onoffshim one-liner, does it poweroff the system if I do a halt (or poweroff) command from the shell? - or does it only do a proper shutdown and poweroff if I press the button on the shim?
I guess my thought was, I could use the Powerboost 1000c as a battery pack/pass-thru charger thingy (so 5v usb power cable from the wall would plug into the Powerboost) then the Powerboost would have a small USB Micro cable going to the onoffshim. If voltage from the battery on the Powerboost drops below some threshold either trigger a GPIO pin to cause a shutdown that the onoffshim is monitoring, or somehow detect it in a script and trigger the shutdown from there.
I have a handheld baby cam monitor (built based very closely to the Pi grrl 2 hardware specs). The only flaw in it really right now is that it will get stuck in a low-voltage boot loop when the battery is too low. And I KNOW that's gonna make for an unhappy SD card if I do that too many times :).
So long story short: yes I would like this to do a safe shutdown when my Powerboost battery gets low.
EDIT: Btw, there are some tutorials online for triggering a halt or shutdown when the voltage drops, but that still doesn't power it off. That's what I think this Shim could be very helpful to achieve is that last final step of powering it off once the shutdown is complete. I will look into that a little bit...