as i remember pnp version of divine might IS charisma mod to damage and duration based on cha mod, so i think this is the way to go, and while at it the tiering of the enchancement should be abandoned.
IF devs dont want splashes to be able to use this enchancement that is given on low lvls, it could have a +25% of bonus every 5 lvls of char, that way lvl 5 pally would gain 25% of cha bonus to damage 50% at 10, 75% at 15 and full 100% at 20, i know there would be big scream about it from all splash pallies but look at it like this, SHOW ME other class that gives so much JUST for splashing in 2 lvls of it.
Soon we are supposed to get new enhancement system, so i hope there will be something done with this enchancement.