I seem to remember Strat suggesting Shadow Adept in a post long ago as an appropriate prestige class in this situation. Personally, I tend to go with Dread Necro 20/Cleric 5/True Necro 15. Yes, I know that is not an optimal True Necro, but it is a great Dread Necro build 39 arcane caster levels (if you take practiced spellcaster), 40 levels of turn undead, 40 levels of necromamtic spellcasting, undead mastery (from DN), familiar (from DN) unique undead summons (from TN), zone of desecration (from TN). However, it sounds as if the newly revised True Necro that advances both arcane and divine spellcasting, won't advance divine spellcasting in this case, because of a Bioware glitch that treats custom arcane classes as divine spellcasters (according to xwarren in another thread). That doesn't bother me too much, since the old True Necro didn't advance divine spellcasting anyway.
All of that said, it never seemed logical to me that DN couldn't progress beyond 20. I'd love to experiment with a straight DN build.