Hi there,
Yes I can verify it does work, ALL the more reason Seeed’s next Xiao should have a PMIC … period , in this day and age to not be able to accurately read the F’n State of charge of the battery, power-cell, nuclear-reactor or Solar connected to the SUN 
, especially when you praport to be a battery powered wearable device solution . Give me a Break. It’s like rule 2 or 3, 
IDK go look it up.
Lucky we have users and fellow makers who take time to figure out solutions and provide them for nothing in return. You don’t have that in any other forum. MARK IT 
All that being said , it works using the Voltage divider, you do give up the GPIO is all. but that would be kinda the same as an INT pin required for a PMIC chip. You get SO much more… Hopefully the Next revision of the Expansion board will have a number of improvements and be usable for all the Xiao’s (even if it requires some jumpers IMO)
Noteworthy you can use the read battery in the older stuff and Non-mbed on the Xiao Directly so there’s that 
Mod is good enough to get you going.
HTH
GL 
PJ 