I used "oblique" merely as another word for the 'objective' or 'accusative' case, i.e., in this example, the form '"me". I put it in inverted commas because I'm not sure it's the best term - but I don't really like the other terms either.
Just a few random thoughts that probably don't explain why "Let us go then, you and I," might sound acceptable:
"The use of "let" is interesting. Special rules may apply when what Huddleston calls the "1st person inclusive let-imperative" is used (particularly in the contracted form "let's"). Traditionally this would be analysed like the normal imperative "let" with "us" as the object, followed by a bare infinitival ("go"), but, as Huddleston demonstrates, it is not treated exactly like a normal imperative (as in "Let her go, will you."). In a different analysis, in dialects that permit sentences like "Let's you and me make it ourselves.", "let's" might be interpreted not as verb+object but as a single element introducing a (finite/non-finite?) clause with a subject and a verb; the subject of a non-finite clause would normally be the default (objective) form ("me", "him", etc.). An example of "let's" in this sense is Eric Berne's psychological "game" "Let's you and him fight". However, I think this analysis would only apply to the contraction "let's", not to "let us".
Suppose Eliot had written:
"Let us fight then, they and we,
When the evening is spread out against the sea. "
I find this less acceptable than "you and I", which makes me think that the (apparent) acceptability of "I" in some cases may be artificial, caused by generations of grammarians insisting on the nominative but always using examples with the first person, meaning that native speaker intuition remained more intact with other pronouns but became "warped" with "I" and "me", until "It is I" actually became part of formal English.
I'm not actually sure that apposition requires case agreement in English but (with few exceptions) I would still expect "me" where the pronoun is not the direct, explicit subject of a finite verb. What about "The committee (the chairman and us) agreed . . ." or "The opposing forces (us and them) were preparing for a long . . ."? Even in German, apposition does not always require case agreement.
I think one problem is that modern English has only a vestigial case system, so syntactic patterns are not reinforced as they may be in other languages. It may occasionally be difficult to distinguish between intuitive rules that are either simpler or more complex than the rules of traditional prescriptive grammar, on the one hand, and problems of "performance" (difficulties in implementing the intuitive rules because of distractions, complexities, etc.), on the other.
Overlaid on all this, there may be a tendency in English (particularly BE?) for semantic, or logical, considerations to override the formal syntactic rules that would otherwise apply, but I don't know if that could really explain "Let us go then, you and I".
It may also be the formal sound, and even the unusual syntax, that we expect from poetry.
Anyway, there are a lot of things that seem almost, but not quite, applicable.