Comment by tom_alexander

> it seems like there's no standard reporting of charge level over the wire

Isn't that the "Battery_Status" message in the USB PD spec? Page 152 in this pdf: https://e2e.ti.com/cfs-file/__key/communityserver-discussion...

Combine that with the "Battery_Capabilities" message (page 163) to get the total capacity, and you can calculate the charge percentage.