If you don't have other load just the Pixl and an INA226, it can run several seconds on a relatively small supercap. Bought a 5.5V / 0.something Farad one ages ago, and that can store enough energy to run the Pixl with LCD backlight on for seconds.
Or modify your Pixl.js to use CR2032 battery as backup:
CR2032 battery backup - the CR2032 can be used as a backup when Vin/USB power is not present. Cut the trace in the solder jumper below the CR2032 holder, and add a surface mount diode to the two pads to the right of it.
So it could even run autonomously on the CR2032 after power loss. Altho 2.7V is the lower limit of the INA226, so that might not work reliably if your battery is low...
I think on your schematic the Din would stay floating around 3.3V, if
+LiPo was disconnected.
If the INA226 doesn't work reliably on battery, I think a simple voltage divider + comparator in the Pixl would be enough. Check then NRF52LL on how to use the comparator.