POSSIBLE SPOILERS

So i accidentally used "Spirit Gorge" near Gann or whatewer his name is (he was near death) and it killed him, then he just ressurected like struck down companions usully do.(He ressurected but was stuck in funny floating animation after that, you can see it on screenshot) After that his essence just appeared in my inventory.
I dont know if it will work with One Of many or with Kaelyn but....

My char have both "Spirit Gorge" and "Devour Soul" feats, then i accidentally used Sprit Gorge on the item on the ground while Gann was standing in the hittzone (my difficulty was full DnD rules, so frendly fire was on) and Gaan was near death so Spirit Gorge hit him and "killed him". And after that i aqired his essence while he was still alive.

Tested it and it seems to only work right after you devour the treeman and aqire "Spirit Gorge" ability, after i tried to rest the ability stopped targetting companions somewhy (yes i lovered their health to neardeath before that).

Both Sofphia and Gaan was killed by it. But it didnt hit One Of Many ( maybe its because i dont have "Eternal Rest" ability).

Tested it again on One Of Many, and even with "Eternal Rest" "Sprit Gorge" wont target him somewhy.
But for amulet you can have Okku's essence anyway.

Tested it after waiting for 5 min repeatedly and it seems "Spirit Gorge" won't hit them second time. Why is that i dont know.

Didnt tested on Kaelin (killed her in Vault) but presumably it must work.

Update, you can dewour Sophia to have her essence when she is possessed bu Ghulkaush in skein.
Dont know if she can posses other companions.

PS. It seems you can infinitly farm Treeman boss for exp and essences as long as you dont kill trees.