I’ve also been curious about reducing the sleep power as low as possible.
When running from the battery, I think the R21 pull-up resistor is wasting about 5uA of power:
4.2V / (806KOhm * 1000) * 10^6 ~= 5uA
Then if you try to disable the voltage regulator by pulling the enable pin low, I think R18 wastes 21uA.
4.2V / (200kOhm * 1000) * 10^6 = 21uA
It looks like the solder bridges could be changed to connect the microcontroller to the VBAT instead of the 3.3V output from the regulator. However, the XC9258 voltage regulator would still be on and wasting 15 to 25uA.
Possibly you could disconnect solder bridges S1 and S5 to disconnect the microcontroller from the voltage regulator, then connect the battery directly to half of S4 (by passing the lipo connector), and connecting S3 to turn on the microcontroller’s built in regulator. However, I can’t see getting a very reliable solder joint between a wire and half of a solder bridge.
The R21, R18, and XC9258 are all under the RF shield. So there’s no easy way to desolder them.
Possibly with a high enough voltage between the EN pin and the LI+ pin (positive here), one could burn out R18 or the XC9258. I don’t have a spare particle xenon to try that now.