Currently the matchbox-panel battery status applet doesn't get built (presumably because of the lack of libapm), it would be nice if we could fix this. libapm is probably obsolete by now, but I'm not sure if the best way to retrieve battery status is to read /sys/class/power_supply/* directly or to use something like upower.
To my surprise this seems to actually work. The "--with-battery=acpi" works and seems to be the choice in usual cases: the applet is definitely built. I don't actually have a yocto test device with a battery, but I've run the mb-panel-2 on my laptop and tested that the applet works; I've also made sure "--with-battery=acpi" is used when building a sato image. Marking WORKSFORME.