There are many mantra drastas (seers) like Kathaka, Mandukya etc in the Vedas.. Are there any seers who are sudras or outcastes?
Yes. Satyakama Jabala is a Sudra who transformed himself into a rushi, according to chAndogya Upanishad. Verses from this Upanishad that describe how he did so, is copied and pasted below. He is an important rushi and his name appears frequently in the Upanishads in the context of at least one particular yagnyA and in context of certain knowledge of the brahman. Upanishads form the gnyAna kAnda portion of the Shruthi, and hence part of Vedas (rushi vAngmayam/rushi vak):
IV-iv-1: Once upon a time Satyakama Jabala addressed his mother Jabala, ‘Mother, I desire to live the life of a celibate student of sacred knowledge in the teacher’s house. Of what lineage am I ?’
IV-iv-2: She said to him, ‘My child, I do not know of what lineage you are. I, who was engaged in many works and in attending on others, got you in my youth. Having been such I could not know of what lineage you are. However, I am Jabala by name and you are named Satyakama. So you speak of yourself only as Satyakama Jabala.’
IV-iv-3: He went to Haridrumata Gautama and said, ‘I desire to live under you, revered sir, as a Brahmacharin; may I approach your venerable self (for the same) ?’
IV-iv-4: Gautama asked him, ‘Dear boy, of what lineage are you ?’ He replied, ‘Sir, I do not know of what lineage I am. I asked my mother; she replied, "I, who was engaged in many works and in attending on others, got you in my youth. Having been such, I could not know of what lineage you are. However, I am Jabala by name and you are named Satyakama". So, sir, I am Satyakama Jabala.’*
IV-iv-5: The teacher said to him, ‘No one who is not a Brahmana can speak thus. Dear boy, bring the sacrificial fuel, I shall initiate you as a Brahmacharin, for you have not deviated from truth’