The Alwars (also spelled Azhwars) are a group of 12 ancient Vaishnava saints who lived in Tamil Nadu and are famous for their devotional poetry in praise of Vishnu. The collection of their 4000 poems, known as the Naalayira Divya Prabhandam, is considered by many to be the "Dravida Veda", or South Indian Veda. The Alwars are crucially important figures in the history of Vaishnavism; it's the beliefs and principles embodied in the Alwars' poems that ultimately gave rise to the Sri Vaishnava sect (of which I'm a member).
Now one of the most famous Alwars was Nammalwar, aka Sadagopan who spent his life under a tree. As I discuss in this answer, in one of his poems Nammalwar says
"I, Sadagopan of Kurugur [am] one who is ... joined with His Holy Feet that grew to the heavens."
Sri Vaishnavas commemorate this by using a crown called the Sadagopam which represents the head of Nammalwar, and it has the feet of Vishnu on top since Nammalwar is united with Vishnu's feet. The idea is that just as Nammalwar surrendered to the lotus feet of Sriman Narayana, so should you.
But my question is, who started the practice of the Sadagopam? Since it pertains to Nammalwar, I assume it was some Sri Vaishnava Acharya. It could be Nathamuni, since he compiled the poems of the Alwars. Or it could be Ramanujacharya, since he often treated Alwar poems as an impetus for action, like going to the Madurai Kalazhagar temple and offering sweets just because Andal had promised to do so in one of her poems. (That's how he earned the title of Andal's elder brother.) It could be even be Thirumangai Alwar, since he was the one who instituted the Adhyayana Utsavam, the first festival in honor of Nammalwar in Sri Rangam.
Does anyone know the earliest Sri Vaishnava work that discusses the practice of Sadagopam? That may help narrow it down. It might have originated in the Upilliappan temple, since Upilliappan is the one whom the Nammalwar verse I discuss above is dedicated to.