Right now it is possible to to scale damage based on sources by configuring each source on a per-string basis. However, undead burning in sunlight internally use the onFire damage type, which is shared with normal fire.

Undead burning for a long period of time is an annoying quirk of the mod, so an appreciated feature would be adding an option which applies additional sunlight damage to burning undead to kill them within just a few seconds of daybreak, similar to the vanilla game.

I've been looking for a way to do this for almost an hour... found nothing. Would be great.

If the issue is still open it has not been implemented.

I am aware, was just saying that this would be a fantastic addition if it were to be added.