In the sentence "Aut ubi muneribus nati sua munera mater addidit, externo marmore dives opus."(Ars Amatoria 1, 69-70), I think "munera" and "opus" are in apposition. If so, "dives opus" must be "divitum opera" taking the same number and case of accusative plural.
If I am wrong, please correct me.
In “Aut ubi muneribus nati sua munera mater addidit, externo marmore dives opus” (Ovid, Ars Amatoria 1, 69-70) the term "munera" (accusative neuter plural) is not in apposition, as it is a direct object depending on the past tense "addidit", while the accusative neuter singular "opus" (work) with its neuter adjective "dives"(rich) is in apposition with "munera" as "opus" adds something more about "munera" and then agrees with this noun in the case.
In short, “externo marmore dives opus” literally means:”a work rich in foreign marble” with reference to the library built by Augustus’s sister Octavia in honour of her son Marcellus (died in 23 BC, nineteen-year-old) after whom Augustus named a theatre, the remains of which are standing still today in Rome.(See also Plutarch, Life of Marcellus,chapter 30,section 6 where we read:“In his honour and to his memory Octavia, his mother, dedicated the library, and Caesar the theatre, which bears his name”)
Please note that the lines 69-70 "Aut ubi muneribus nati sua munera mater addidit, externo marmore dives opus" literally mean:
”...or(AUT) [you can have a walk] where (UBI) the mother (MATER, i.e.Octavia) added (ADDIDIT) to the building (MUNERIBUS, i.e. the theatre of Marcellus) of her son (NATI, i.e.Marcellus) her (SUA) building (MUNERA, i.e. a library) , a work (OPUS) rich in foreign (EXTERNO) marble (MARMORE)", i.e.:
"...or [you can have a walk] where the mother added to the building of her son her own building, an edifice rich in foreign marbles".
As for the term MUNERA (from MUNUS), I have to point out that, when used in the plural,it means:” public building for the use of the people, erected at the expense of an individual”, just like the library and the theatre to which Ovid refers.