Since the undead by AD&D rules can't be healed with Healing sphere spells, I'm going to have to look into alternative ways of conferring hit points on them. The reverse of Cure spells, Cause Wounds, should restore them. There is no Cause Light Wounds or Cause Moderate Wounds in BG:EE, though Spell Revisions and other mods like that might have added them. In any event, those spells are going to have to be modified and the undead receive an immunity to the Current Hit Points Bonus effect, because that effect would cover cures for both the living and the undead. The alternative is to edit the existing Cure spells so that they deliver their healing only to non-undead creatures and separately fix the Cause spells to harm everyone but undead (and golems etc.) and heal just the undead.
If I go for the option of blocking Current HP Bonus for the undead, I could use Damage (12) with a negative number as a variant cure. But it gives a message that such and such was "immune to my damage" after it heals. Not a big deal, but still. Any more ways to heal outside of CHB and Damage?