The service rendered by Periya Nambigal for Maraneri Nambi is not found in our Guruparampara. It appears that even Svami Desikan has not dealt with this. Even if Periya Nambi has performed such a kainkaryam, it should be as per the strictures of varnashrama dharma as he never violated the ordains of varnashrama dharma even a minute; Svami Desikan has said in the Srimad Rahasyatrayasaaram, that he strictly performed all the ritualistic austerities.
Even though none of his relatives were Srivaishnavas, Maraneri Nambi was a strict Srivaishnava. He had a desire that his body must not be given away to his relatives for last rites and he had requested for this. Given this, certain rites can be performed within the boundaries of scriptures.
Hence, last rites for Maraneri Nambi might have been performed conforming to the scriptural and varnashrama dharma requirements. If no relative is available to perform last rites, scriptures allow others to perform