From The Apple Wiki

GlyphPlugin is located at /Firmware/all_flash/all_flash.(MODEL-NAME).production/(GLYPHPLUGIN-IMG3-FILE-NAME).im4p in the IPSW and flashed to NOR (or a functionally equivalent partition). It is displayed when the battery is too low for the device to boot up (under 5%)and external power is not connected.

It is an iBootim image packed into an IMG4 (.im4p) archive. No (user-friendly) method of replacing this icon is known.