You could do the Warmage/Rainbow Servant for Cha based casting with all cleric spells available, then gestalt for max Cha on the other side.
As a gestalt character, playing a Warmage wouldn't be so bad if you covered your bases with say Chameleon casting on the other side. Plus Int as the secondary stat works for both sides.
Human
Warmage 6/RS10/Warmage 7-10 //Beguiler5/Chameleon10/DragonDevotee1/Fatemaker1-4
You need two feats, Able Learner before 5th, and a Skill Focus before entering Fatemaker.
So at 16th level, you cast as a 16th level Warmage with full spontaneous Cleric spell access (Cha+Int), a Beguiler 5 (int), and a Chameleon 10* (Int or Wis; CL=2x class level). Fatemaker adds a small amount of Cha based casting also.
With a +9 to cha from PrCs by 17th. (+2 Dragon Devotee1, +1 Fatemaker 1 , +6 as a daily boost from Chameleon 10). Add in your +5 from levels for a total of +14 by 20th
*Chameleons can take 2 focuses at 7th level. That means you could have Arcane Spells (Int) and Divine Spells (Wis) (1-6th level each @ 20th CL by 10th class level.) I would also argue that you get 2 separate +6 bonuses to different stats, since you get the bonus when you adopt a focus, and at 7th level you adopt 2 focuses each day, so that would by +6 to Cha, and +6 to Int.