I was planning on using NWNX_fixes to limit ability increase stacking to 5 (so it would be more like 20 damage over 7 or 8 rounds), and changing Divine Might and Divine Shield to their 3.5 equivalents (respectively: +CHA bonus to weapon damage for 1 round, +CHA bonus to Shield AC for CHA bonus rounds). I will most likely use the Shadows of Tilverton hak and script base for those modified feats. This would considerably offset the increase in smite damage, and should make it roughly balanced. Divine Might and Divine shield would become less imperative as feats, but still very useful, especially at low levels.
If there's a way of limiting the smites to a single target while keeping the duration, that would be ideal for balance, since you wouldn't want a paladin with Great Cleave stomping everything in the immediate vicinity every time they dropped a smite. A script that triggered a smite on the initial target at the beginning of each round would be ideal, though it would need to check that the target was not dead, of course.
Update: I took another look at the SoT script base, and it should be fairly easy to modify their Smite Infidel script's duration and damage. The only thing I'm not sure how to do now is limit the target to a specific target creature.