Carrageenan, a seaweed extract, is used frequently in vegan cheese and other vegan and vegetarian preparations as a thickening agent.
However, I have never seen it recommended as a way to add "body" and texture to a vegan stock, as a replacement for the collagen in meat stocks. Particularly, I'd like to experiment with it for vegetable stocks intended to be thickened into a sauce.
Is this because it works poorly for that purpose, or just because few cooks have tried it? Or some other reason?
Carrageenan is not suitable for the application you propose.
First, you need iota carrageenan, since the kappa variety is inhibited by salt.
Second, you need a source of calcium ions for the iota carrageenan to form a gel (kappa needs potassium). I doubt that it will be sufficient to chuck a piece of kale in there, you will probably need something more. I don't know for sure how it is done in vegan cheeses, but this type of preparation is frequently firmed up with inorganic salts. If you were to add calcium sulfate to a stock, you would end up with something rather unpleasant, I imagine.
Third, the carrageenan action is also inhibited by long heating at low pH. A stock is usually not too low in pH, but it is lower than neutral.
In short, my suggestion is to pick a better suited thickener.
At such a low dose as in stock, guar and xanthan will suspend fat better and give a silkier feel that carrageenan.
However, Guar and Xanthan are harder to mix in without clumping and can easily be over used ie slimy.
If clarity is not an issue, a puree of sauteed onion is silky addition to add body.
