What I was getting at is that this looks like complete moral relativism -'right for me' is the only right there is

While it is relativism, the focus is a bit different from 'right for me.' More like 'this action measures up as right against standard Y' where this Y is typically something I endorse.

For example, if you and I both consider a practice morally right and we do so because it measures up that way against the standard of improving 'the well-being of conscious creatures,' then there's a bit more going on than it just being right for you and me.

Or if I consider a practice morally right for the above reason, but you consider it morally wrong because it falls afoul of Rawls' theory of justice, there's more going on than it just being right for me and wrong for you. It's more like I'm saying it's right{Harris standard} and you're saying it's wrong{Rawls standard}. (...at least as far as cognitive content is concerned; we would usually also be expressing an expectation that others adhere to the standards we support.)

Of course the above are toy examples, since people's values don't tend to line up neatly with the simplifications of philosophers.

(since you seem to be implying there is nothing interesting to be said about the process of negotiation which occurs when people's values differ).

It's not apparent that values differ just because judgments differ, so there's still a lot of interesting work to find out if disagreements can be explained by differing descriptive beliefs. But, yes, once a disagreement is known to result from a pure difference in values, there isn't a rational way to resolve it. It's like Luke's 'tree falling' example; once we know two people are using different definitions of 'sound,' the best we can do is make people aware of the difference in their claims.

I had in mind theories like Utilitarianism or Kantian ethical theory, which are general accounts of what it is for an action to be good, and do not aim merely to be accurate descriptions of moral discourse (would you agree?).

Yep. While those are interesting standards to consider, it's pretty clear to me that real world moral discourse is wider and more messy than any one normative theory. We can simply declare a normative theory as the moral standard — plenty of people have! — but the next person whose values are a better match for another normative theory is just going to disagree. On what basis do we find that one normative theory is correct when, descriptively, moral pluralism seems to characterize moral discourse?

Is it possible for a rational person to strive to believe anything but the truth?

If being rational consists in doing what it takes to fulfill one's goals (I don't know what the popular definition of 'rationality' is on this site), then it is still possible to be rational while holding a false belief, if a false belief helps fulfill one's goals.

Now typically, false beliefs are unhelpful in this way, but I know at least Sinnott-Armstrong has talked about an 'instrumentally justified' belief that can go counter to having a true belief. The example I've used before is an Atheist married to a Theist whose goal of having a happy marriage would in fact go better if she could take a belief-altering pill so she would falsely take on her spouse's belief in God.

Or, to put the point another way, isn't having truth as its goal part of the concept of belief? [...] But if this is fair I'm back to wondering where the ought comes from.

Perhaps it comes from the way you view the concept of belief as implying a goal?