For an example of this, in some of the settings that I've run games in, I use fantastical life-cycles for non-human creatures.
Cool.
The point that I'm trying to make is that within your societies, you need rules that makes sense for establishing what a "person" is. And further, that these fantasy societies, because they are animistic societies where every cow and tree is in some sense an aware and thinking being, they are going to like animistic societies have developed complex graduations of "personness" and various degrees of "person rights" depending on the order of being they are dealing with. Trees are going to have "tree rights". Cows are going to have "cow rights". When a woodcutter cuts down a tree, he's going to need to have a good reason, and he's going to be required to explain to the tree that now is the time, that his family needs wood, but that he promises to be a good caretaker of trees and ensure the tree has many offsprings and cousins. The rancher when he slaughters a cow is going to operate in the same manner, and so is the hunter that shoots the deer.
Everyone is familiar with the Declaration of Independence, and in particular the line that goes, "We hold these truths to be self-evident, that all men are created equal, and endowed by their creator with certain inalienable rights." It may have been a bit of bravado to declare the self-evidence of that in the real world, but in a fantasy world it really is self-evident that everything is endowed by their creator with certain rights. The question then becomes, who is that creator, do you respect their wishes, and what are those rights if you do?
If everything has the same creator, and was created equal, then you have to respect them equally no matter what they look like - elves, dwarves, gnomes, kobolds, gnolls, goblins, bullywugs, doesn't matter. That makes perfect sense. The creator (maybe?) says, "I created you all equal, no treat each other that way. Respect the rights and dignity that I have granted you." If your campaign world is set up that way, or if you have some sort of equivalent creations (all the races were born from the spilled blood of the gods during some primordial war, all the races sprouted from the pollen that flowed from the tree of life, or whatever) then an organization advocating for equal, fair, and just treatment toward everyone in a group of peers makes perfect sense. It's a philosophy with real intellectual heft to it and it's application is, perhaps difficult at times, but in theory pretty straight forward.
On the other hand though, the default D&D setting does not have equivalent creations. Gnolls for example are the progeny of some sort of a single demon lord, and exist to further that demon lords interests. They are fancy puppets or AI created by a literally demonic designer. Do you really need to respect that designers wishes, and respect the rights that designer bestowed on his creation? It really doesn't make much sense to unthinkingly say, "Yes!" to that proposition. Maybe occasionally one gnoll goes rogue due to a system error, and you treat that gnoll differently, but in general "gnoll rights" are something you don't have to respect and you respect that one rogue gnoll precisely because they aren't really a true gnoll anymore.
Sentience simply can't be the sole guide (or soul guide?) in an animistic setting. And heck, it's only the sole guide now in the real world because we only have one sentient species to extend friendship toward. (Note that we have "human rights", but that we accord lesser but still important rights to our semi-sentient cousins the animals - "animal rights". And we argue over that, explicitly or implicitly according to the nature of the creature.) Once we start creating sentient species, or if we were ever to encounter others, all these questions and problems will be laid in our lap just as they already are in a fantasy setting.
Ghouls? Sentient, yes. Person... no?
Dragons? Sentient, yes. Person... maybe? Probably depends on how you've envisioned dragons. Do Tiamat and Baphomet exist, and did one create all of one group, and one the other?
Glabrezu? Sentient, yes. Person, no.
Mind Flayers? Sentient, yes. Person... you get the picture.
In my setting I laid out a cosmology where the origin of things defines the degree of personhood involved. And as I've argued elsewhere, where things get fuzzy, reasonable people in the world disagree - sometimes for good reasons and sometimes for bad ones.
To really support a broad view of personhood in D&D you'll need to really reinvent the setting. The broader you want to make it, the more you'll need to reinvent it - especially if you want to have something that is more than a superficial gloss that is ignored in practice. For example, you certainly could have ghouls have personhood, but you'd need to really work to make that meaningful. You'll also have to deal with problems that will inevitably arise. For example, why should a dragon, if they are a person, merely have the same rights as a human, seeing that dragons are wiser, more intelligent, and stronger than humans. From their perspective, shouldn't "human rights" be as much less than "dragon rights" as "dog rights" are less than "human rights"? If this isn't true, why isn't it true?
