Two additional thoughts about the Heavenherds... and then some more.
Some people may wonder how the heirs to Thera/Atlantis, specifically trying to preserve the lore and knowledge, could end as a tribe of hunter gatherer instead of, say, a major civilization building pyramids or the Nuremberg stadium. I got an answer for that: magic. Thera depended on magic. Everything from they do in from mining to architecture would heavily relies on magic. Remove magic and all that knowledge is next to useless.
On top of that, maybe the Heavenherd were also actually given instructions to keep a low profile during the whole Fifth Age, since major civilizations tend to get involved in war and all that.
Let's also go with the crazy theory. We're told about the Heavenherd in Augmentations because members of the tribe used the live-lengthening rituals as a basis for cybermancy in Universal Omnitech project. Universal Omnitech, as you may know, is a biotech corporation, headquartered in Vancouver but also very active in Africa (after moving into the synthetic diamonds market with Dikote, they came to bought off De Beers).
There is another organization that deal with biotech, active in Africa, whose very name points at an origin in the Pacific Northwest: Tamanous. We can imagine there would some connection between one major biotech corp and the major organlegging gang operating in the same business and the same areas.
For the Pacific Nortwest tribes, the Tamanous is the name of one or several spirits, depending on who you ask (as far as I understand, near California and Oregon, legends refer to Tamanous as a single great spirit, while in Wasghington and British Columbia Tamanous is a generic term). Canadian law did ban at some point the potlatch rituals as well as the Tamanawas dance. The SR organleggers organization may be related to the Hamatsa cannibal secret society of the Kwakiutl tribe that worshipped a spirit called the Black Tamanous (on the other hand, considering the size of the Kwakiutl tribe and its poor condition within the Tsimshian police state, it may be unlikely Tamanous origin would be such a secret if it was a purely Kwakiutl thing). Potlacht is one forme of ritualized material sacrifice (well, not so ritualized in SR when the Tsimshian nation used to apply death penalty on Potlatch Day...). Cannibalism on the other hand, is straight blood magic. To put it another way, Pacific Northwest culture share some traits with Mesoamerican cultures.
So, we can imagine Tamanous and Universal Omnitech would pick up their shamans from the same lodges, lodges that practice blood magic. And the Heavenherds would have worked with such shamans to perform cybermancy... We can go as far as claiming that of all the magical groups that exist in the world, not the least of them the two elven nation that wooed them, the Heavenherd chose to work with blood mages from the Pacific Northwest. And at that time, UO was also cooperating with Aztechnology...