Dear Mathew,
Since you brought the topic of Narayana guru, I thought you were trying to understand the textual , both documented oral texts and manuscripts that might help in understanding the background that is behind the existence/emergence of a Narayana Guru .
So I was pointing to two such references : 1. Oral texts that are collected/documented and not yet documented. 2. Manuscripts that reflect aspects like Hatha Yoga -Advata Vedanta interface (Advaita Vedantic Hatha Yoga or Hatha Yogic Advaita however it can be labelled) etc.,
You seem to see the 'Bhakti movement' framework sufficient to handle the issue. An aspect that did not receive sufficient attention is what can be called as 'Yoga Movement', that has similarities and differences with 'Bhakti movement'. This movement has two main strands : 1. Yoga-emphasizing, that has features strongly differing from Bhakti in aspects such as opposing Murti-puja etc. 2. Yogaizing Bhakti , with esoteric/mystic interpretations of Bhakti material.
Now that you mention your interest in S'astric writings towards this, let me share that the concept of 'Shastric' itself needs a more nuanced look. For example, Advaita Vedantic Hatha Yoga or Hatha Yogic Advaita texts look at themselves as Shastric texts only. Even these texts may not have any social discussion. But it is these texts and their ideas that reflect in a big way in the oral and rarely written texts of the Yoga movement involving the scheduled castes among other non 'upper caste' social groups.
The other Advaita Vedanta texts of the same period may not take up this question, as they focus on other core philosophical , non-social questions of Vedanta.
What deserves attention is that these oral 'folk' lyrical and non lyrical texts do have a significant portion devoted to this question of adhikaara and the discussions therein view themselves as shastric only , use shastric terms and logic. There are legends within the lore of these 'Yoga movement' communities, about how their leading Saadhaka, the parama guru of their parampara, 'defeated ' a Shastric scholar or Shastric mathaadhipati in a Shastric debate on topics that may include questions of adhikaara. Their lyrical and non lyrical lore banks upon provisions such as antaryaaga , inward practice interpretations of Yajna and other Karmas available in Upanishads , relative greater significance of jnaana over karma found in Vedanta and so on and so forth, all from the Shaastra of Vedanta and its most core texts Upanishads , Bhagavadgita etc., only. While the Hatha Yogic Advaita Vedanta texts that seem to have influenced these thoughts are in Sanskrit , all these lyrical and non lyrical 'folk' texts are in the vernacular/ regional languages.
At another level, people like Swami Dayananda Sarawswati consider their arguments in favour of adhikaara of all humanity including the scheduled castes of India and all the people of the world for both the Vedic Karma kaanda and Vedanta, view their arguments as Shastric only and they are in Sanskrit only.