Seems pretty straightforward to run a script from EmulationStation. Proof of concept, here is a crude 10-minute version I made using Hursty's "Bluray" theme. It already contains a "Power" system, so I just added a small script, uploaded a small .png of the DualShock controller and edited the gamelist.xml file.

MAIN MENU (from Hursty):
main menu.png

SELECTION:
selection.png

OUTPUT W/Pause (currently requires a keystroke instead of a button-press, but can modify this):
ps4power-2020.04.14-17.24.46.png

The simple script:

#!/bin/bash clear echo PS4 Dual-Shock Battery Level: cat /sys/class/power_supply/*/capacity echo read -p "Press Enter to continue"

Be sure that the script is set as executable.

Lastly, the es_settings.cfg entry for the "Power" menu item to appear:

<system> <name>power</name> <fullname>Power Menu</fullname> <path>/home/pi/RetroPie/roms/power</path> <extension>.sh .SH</extension> <command>%ROM%</command> <platform>linux</platform> <theme>power</theme> </system>

Ideally, the output would be prettier and require a DualShock button to be pressed instead of the keyboard Enter key, but this will certainly work.

PI-Top Retropie battery monitor/display??