So I have a character who has a tattooed headband of mental prowess. Dose his modified Cha effect his leadership score?
Assuming that said headband actually does affect Charisma (ie if it's an Int/Wis headband, it will do precisely diddly), then yes.
Temporary Ability Score increases affect everything that a permanent ability score increase affects, but don't change the ability score until 24 hours after the increase is gained. Even so, gaining +2/4/6 to your Charisma increases your Cha modifier by +1/2/3, and since:
Your base Leadership score equals your level plus your Charisma modifier. In order to take into account negative Charisma modifiers, this table allows for very low Leadership scores, but you must still be 7th level or higher in order to gain the Leadership feat. Outside factors can affect your Leadership score, as detailed above.
then it definitely affects it.