First, full disclosure: I work for mophie. I don't want to mislead anyone, but I noticed this post in my daily Google Alerts today, and thought I could help out. I saw that jobosumm said:
And there isn't any way to get rid of the battery symbol during charging.
I believe that jobo is correct that you cannot default display your wallpaper when your phone is charging; however, I'm not certain that is what dayoungtexas meant.
Second, is there a way to change the setting so that when the phone is in the case the battery meter isn't showing, your wallpaper is?
The answer to dayoungtexas' question should be yes. There is an on/off charge button on the Mophie case. Your phone will only show the charging logo while the charging trigger is set to on. If you flip the switch to off, your phone is not charging, so it can display whatever your default desktop would be. Once your phone's internal battery gets low, you can then flip the switch on and your phone will charge up. Once you're happy with the charge level, flip the switch back off and you're right back in default mode.
I hope this is actually what you're asking and that I didn't simply misunderstand your question. Hopefully this helped. Have a good night.