I don't think they do have any "special metaphysical status," and indeed I agree that they are "simple social conventions." Do I make statements about moral rights and wrongs? Only by reference to a framework that I believe the audience accepts. In LWs case, this seems broadly to be utilitarian or some variant.

That's precisely my point--morality doesn't have to have any metaphysical status. Perhaps the problem is simply that we haven't defined the term well enough. Regardless, I suspect that more than a few LWers are moral skeptics, in that they don't hold any particular philosophy to be universally, metaphysically right, but they personally value social well-being in some form, and so we can usually assume that helping humanity would be considered positively by a LW audience.

As long as everyone's "personal values" are roughly compatible with the maintenance of society, then yes, losing the sense of morality that excludes such values may not be a problem. I was simply including the belief that personal values should not produce antisocial utility functions (that is, utility functions that have a positive term for another person's suffering) as morality.

Do I think that these things are metaphysically supported? No. But do I think that with fewer prosocial utility functions, we would likely see much lower utilities for most people? Yes.

Of course, whether you care about that depends on how much of a utilitarian you are.