It was originally layered on a Marshal build, where the goal was tanking Charisma, using a... generous interpretation of the "Charisma bonus (if any)" that the Marshal has.
After all, as far as I can see, Charisma bonus does not have a unique definition, and can be taken as a synonym of Charisma modifier.
She used Inspire Charisma to give herself a +Cha mod penalty on Charisma checks.
I also was considering stacking on Vow of Peace, just so that her "allies" took penalties for killing things.
I'm kinda sad that none of the other Diplomacy feats do anything fun if you fail them.