Did Ramanuja introduce the Namam? What did Vaishnavites wear before that?
The Urdhvapundra has been worn from ancient times (pre-Ramanuja). The Pancharatra, Puranas and Smritis glorify the Urdhvapundra worn with white clay and gopi-chandana.
In the Sri Vaishnava Guru Paramparas, when discussing the life of Ramanuja, it is recorded that the white clay worn as Urdhvapundra was difficult to obtain in those times. Ramanuja managed to identify a place in Melkote where it was available.
I am not aware of an English translation of the Srivaishnava Guru Paramparas which is traditionally in Manipravala but the story is mentioned in the various English biographies of Ramanuja. E.g. See the chapter 29 on Melkote in Alkondaville Govindacharya's version - https://archive.org/details/in.ernet.dli.2015.214575/page/n189
Vedanta Desika analyzes various pundras in his Saccaritra Raksha chapter 2 and reasons why a Mumukshu should only wear a vertical pundra.
His citations in support of wearing the Urdhvapundra span Shruti (Mahopanishad, Katha Shakha, Bhalla ShAkha (also called Bhaallavishaakhaa in Sureshvara's Vartika), and Atharvana Veda), Smritis (Agniveshya Grihya Sutra, Bodhayana, Vriddha jAbAli, shAtAtapa, Vishnu Smriti, Daksha), Puranas (Matsya, nAradIya, Brahmanda), Agamas (pArameShtya saMhitA, sanatkumAra saMhitA, brahmANDa saMhitA, vAmana saMhitA, pArAsharya saMhitA, bhArgava saMhitA, brahmarAtra, nAradIya)
When discussing the materials used in the Urdhvapundra, Desika cites ShankhaH, Gautama, Apastamba, Shatatapa, Atharvashiras, Varadaguru's Tattvasara, Brahmanda, Markandeya, Parameshtya Samhita, Kundina's Krishnamantra Kalpa, Brahmaratra, nAradIya saMhitA, Vriddha Vasishtha Smriti, pAdma saMhitA, pArameshvara saMhitA, Aditya Purana, Varaha Purana, Markandeya Purana, sAtvata saMhitA.
Desika also cites the works of the Azhvars, in particular Nammazhvar's Tiruvaymozhi 4.4.6, 4.4.6, 4.5.6, 4.6.5.
When discussing the location where the clay should be gathered from, Desika cites pArameShTya saMhitA and brahmANDa.
There are other topics discussed including the specifics of the type of clay, its color, its shape, how to apply it, etc. Largely the same sources discussed above are cited in these sections.
Among the clearly older texts, the Mahopanishad, Agniveshya Grihya Sutra, Sattvata and Padma Samhitas, Vishnu Smriti and the works of Nammazhvar are all clearly pre-Ramanuja.
Citing the Mahopanisad verse
dhṛtordhvapuṇḍraḥ parameśitāraṃ nārāyaṇaṃ sāṃkhyayogādhigamyam । jñātvā vimucyeta naraḥ samastaiḥ saṃsārabharairiha ceti viṣṇum
dhṛtordhvapuṇḍraḥ jñātvā ityanvayaḥ। mumukṣur naraḥ dhṛtordhvapuṇḍraḥ dhyāyet iti vidhyarthaḥ।
This is to be understood as "Having worn the Urdhvapundra, having realized (Narayana)... is liberated". The injunction means - A man desirous of mokSha should wear the Urdhvapundra and meditate.
Some other references cited by Desika:
Katha ShAkha says that a noble soul is one who wears the Urdhvapundra (among some other attributes).
dhṛtordhvapuṇḍraḥ kṛtacakradhārī viṣṇuṃ paraṃ dhyāyati yo mahātmā
svareṇa mantreṇa sadā hṛdisthaṃ parātparaṃ yanmahato mahāntam
The Atharvana Veda says that one who wears the Urdhvapundra in the shape of the foot of Hari (eventually) attains liberation.
hareḥ pādākṛtimātmano hitāya madhye cidramūrdhvapuṇḍraṃ yo dhārayati sa parasya priyo bhavati, sa puṇyabhāgbhavati, sa muktibhāgbhavati
Agniveshya Grihya Sutra declares that the Urdhvapundra should be worn and the Tripundra should not.
tripuṇḍraṃ brāhmaṇo vidvān līlayāpi na dhārayet । dhārayetprayataḥ samyagūrdhvapuṇḍraṃ tu nityaśaḥ