"How do you justify ‘objectively valuable’ while restricting discussion to a ‘human universe of discourse’? Does this mean that ethics is not relevant to our interactions with anything that operates outside of this ‘human universe of discourse’? How does this impact on environmental ethics and the value of certain ecosystems?"
A Great question! No, for the universe itself is inside the human universe of discourse and our planet is the part of the universe within our immediate interests. Let me explain how Adaptive Pragmatism comes at the grounding problem in ethics. "There is no ‘objective’ value, no uniform treatment of what is considered eternally valuable in the world. This doesn’t render ethics useless as such, but it does require us to look at structuring ethics in a similar way to how physics is working to structure time: as an additional dimension to reality that is relative to one’s experiential position in the universe." Firstly, it accepts this statement you have made to be the one that is really objectively true.
When I say "within a certain universe of discourse" I am employing one of the tools we use to talk about fiction. "Harry Potter is a wizard" and "Harry Potter is a cat" in reality are both equally untrue. However within it's universe of discourse wherein we temporarily acknowledge JKs imagined reality as a subsisting one we can discuss, the former statement is true, "Harry Potter is a wizard".
Back to your statement about Relativism; Before us, there was no ethics, good, evil, grey, value and meaning. We created those concepts. Now, in order for us to structure ethics we first need to think about it's modality. What is its function, how functional is it currently, how functional it can be and how functional it cannot be? If we say "it's function is to collectively keep humanity safe for as long as possible" then we have to reject any utility in relativism. (I know, crazy right? To reject Relativism because it is true. It isn't pragmatic though.) Now, in pragmatic ethics it's not really individuals who are to be considered moral or immoral it is the society they create that is being judged. The application of Normative ethical relativism to the lay person tends to go something like this. "You can't say x about culture y, that is just how they do things, you can't tell them they are wrong." So; A) there are no universal norms and B) ideas of moral right or wrong are relative to the society in which people are raised and in which they live. Doesn't B sound a lot like a universal norm? Descriptive ethical relativism is fair game as it's utility lies in describing the ethics of ours and others cultures in a more wholesome manner. There is really no compelling argument to make use of relativism as a prescriptive ethical methodology because it sheds no light on what we as individuals should be doing with ourselves.
So the ethics of adaptive pragmatism are grounded in a function of ethics. I define the function is to collectively keep humanity safe for as long as possible, so I start to look toward science. Does Science say not to fuck up the ecosystem if you want to live? Then don't do that or try to unfuck it. Does science say inflicting abusive traumas on people may make them crazy, unpredictable and dangerous potentially to you or people you care about? Then don't do that. Does Science say we need to properly manage our environmental, economic and moral ecosystems in order to thrive and survive? Then manage that shit! Can pragmatic ethics be defined by theory alone? No, but plenty of moral agents out there are acting out experiments in morality unknowingly and knowingly and they can be observed in order to learn more.
Then we have the "What it could be?" question. Imagine if you will that dogs and cats are starting to evolve similar cognitive abilities to us and starting to engage in meaningful language, maybe they even develop functioning thumbs. It is my personal belief that I cannot know everything there is to know and some things I am predisposed to never know because of who I am, so a bigger and more diverse collective can know more and our science benefits from having more knowledge. So because it is Adaptive pragmatism in this imagined scenario the focus would shift from the human perspective toward an earthling perspective (That is sentient being from earth) and how it would adapt at meeting an alien race would be dependent upon their temperament and reaction toward us.
So to summarise my answer to your question; Objectively valuable is whatever has the best means for carrying out the objective of moralities function.
This doesn't mean we throw out anything that isn't currently useful to us either, as philosophy is useful to us even when it moves outside of the human universe of discourse and starts to look at pure truth again, not human truth. We are human though so we have to acknowledge there is a complex difference between Pure Truth and Human Truth.
The methodology behind AP is what initially led me to believe that my intolerance of the ableists who discriminate against myself or others, as an emotional tool which would make them understand the folly of their ways. It also enabled me to listen to you when you supplied a better perspective which enabled me to see more value in temperance again.
https://thephilosophyforum.com/discussion/3564/why-support-only-one-school-of-philosophy
In regards to your discussion here and what you have described: You May find this helpful as AP is similar to what you describe. An all encompassing view of philosophy with the perspective that it is a toolbox.