But since we allow tables and schemas with dotted names in them, I'm uncertain what \d foo.bar.baz is really asking.
FWIW, it’s absolutely clear to me that "." is a special character which has to be quoted in order to be in an identifier. In other words, a.b.c is three identifiers separated by two period punctuation marks; what exactly those periods mean is another question. If somebody uses periods in their names, they have to quote those names just as if they used capital letters etc.
But that's just my impression. I comment at all because I remember looking at something to do with the grammar (I think I wanted to implement ALTER … RENAME TO newschema.newname) and noticed that a database name could be given in the syntax.