While the idea of 'good' undead seems quite plausible to me, the very thought of it taking the form of angelic/spiritual beings ruling over a bunch of humans is rather... silly. To be honest, it would be extremely hard to say it's something new, as we already have civs that are commanded by angelic-like createrus (Basium, or Grigori). I would approach it from entirely different ways.
From my point of view there two main problems of such a civ:
1. The morals of theirs would be something completely unique and maybe a bit hard to grasp
2. They would have hard time being accepted by other 'good' civs (and since they would consider themselves good after all, others as well).
Point 1:
The primal community from which the civ derived had unusual beliefs, based on some strange occurances. Maybe it's members were very strongly linked to death magic and even subconciusly could use it for their own good - and I don't mean the individuals, but community. It was natural for them that once someone dies his body should be animated, yet treated no longer as a person but automaton, capable of doing simple tasks. Those people could believe (or even indeed know), that the soul left the body only to be reborn later on, therefore no mourning would occur and they would feel free to use those 'zombies' to do manual tasks or defending the community (no pain and the ability to fight on no matter what making them very valuable soldiers).The main problem would be to make such a husk not fall apart, so it would require many embalming rituals performed on regural basis (thus making it hard to maintain a large number of such creatures).
This may seem extremely abstract to some, but basically it comes down to the basic system of principles this community would have. Death being no big deal to them and raising the dead coming natural.
Now, how could such a radical civilization could come from? I'd believe that they could be people similar a bit to the Doviello, yet absolutely different. Having to live on the surface during the ice age they had to find a way to survive but instead of pillaging they adopted a way of using any resource availible. And the main resource were the dead. They might even bear an animosity towards Diovello. When the ice age ended they had no important stimulus to cease practicing their ways and therefore they didn't.
So, what I'm proposing is a civ that would resent trapping immortal souls but find using the carcesses and corpses as extraordinairly efficient and acceptable way of getting manual labor (and soldiers).
Point 2:
When other nations started to emerge they must have been quite startled by a community of the living who freely practice raising of the dead. The lack of 'proper' respect for the fallen would even be a shock for some. But seeing that those people were very open-minded and didn't tend to warmongering at all they could be accepted. There still seems to be the penalty in diplomacy for using death magic (ie. having a death mana node) and I think this would perfectly represent the unease of these people toward this 'undeady' civ. Still, those who would have a more evil approach of using the dead and/or the living would meet with utter disrespect of those peaceful people, who would tend to care for their lives and for those around them.
Please notice that this not a full civ but just a few basic concepts for them and should be treated as a fundamental for a true civilization rather than a finished idea.
Btw, there was a kind of undead non-evil society in Planescape: Torment, where you could freely talk with it's members (who were zombie, as far as I recall), but I don't think they were good as well.
Cheers
