PV Kane in his History of the Dharma Sastra Vol 2 Part 1 Page 93 says
There is no taint of untouchability when a person is touched by an
untouchable in a temple, religious processions and marriages, in
sacrifices, and in all festivals’ (Atri verse 249). Sātātapa
quoted in the Smriti Candrika declares that there is no doṣa in
touching untouchables in a village on the public road or in a
religious procession or in an affray and the like, and also when the
whole village is involved in a calamity. Brhaspati also remarks that
there is no fault and so no prāyaścitta if one comes in contact
with untouchables at a sacred place, in marriage processions and
religious processions, in battle, when the country is invaded, or when
the town or village is on fire. The Smṛtyārthasāra summarises the
places where no blame in incurred on the ground of mixing with
untouchables viz. in battle, on public roads leading to a market, in
religious processions, in temples, in festivals, in sacrifices, at
sacred places, in calamities or invasions of the country or village,
on the banks of large sheets of water, in the presence of great
persons, when there is a sudden fire or other great calamity. It is
somewhat remarkable that the Smṛtyārthasāra speaks of untouchables
entering temples. Nirnaya-sindhu quotes a passage of the Devī-Purāṇa
that expressly authorizes antyajas to establish a temple of Bhairava.
There is no traditional justification for not allowing Chandalas to enter into Temples. According to Vaishnava Puranas and Agamas, its forbidden to practice untouchability against devotees irrespective of Caste.
There are no distinctions among such pure devotees in terms of social
class, education, bodily beauty, family status, wealth, occupation,
and so on. (Narada Bhakti Sutra 72)
Yogis are born among all castes; and no caste-odium shall hold in
their cases, for they have seen their souls.
(Bharadvaja-Samhita,1.44).
Even a person born in a family of dog-eaters immediately becomes
eligible to perform Vedic sacrifices if he once utters the holy name
of the Supreme Personality of Godhead. Even if born in the families of
dog-eaters, such persons are worshipable. Persons who chant the holy
name of Your Lordship must have executed all kinds of austerities and
fire sacrifices and achieved all the good manners of the Aryans. To be
chanting the holy name of Your Lordship, they must have bathed at holy
places of pilgrimage, studied the Vedas and fulfilled everything
required. (Srimad Bhagavatam 3.33.6–7)
A Chandala who is a pure Vaishnava at heart and who decorates the
eleven parts of his body with Vishnu-Tilaka and sandalwood paste is
always worshipable, even by the best of Brahmanas. (Padma Purana,
Uttara Khanda 66.84)
A Bramhana who is expert in studying all four Vedas is not dear to Me.
But a devotee who comes from a family of Chandalas is dear to Me. He
should be offered gifts and one should accept gifts from him. He is as
worshipable as I am. (Hari-Bhakta-Vilasa 10.127)
If a devotee of Vishnu, even though he be a Chandala, is recollected,
named, seen or touched accidentally by anybody, O dear madam, the
former purifies them. (Varaha Purana 211.9)
Oh Lord at Sri Rangam! Even brahmins in the top caste, Learned in all
the four Vedas and their six adjuncts, will become outcastes, should
they decry Your devotees in the low caste”. (Thirumaalai,43)
It is through deep love that the Chandala makes me his own, while
without it the Brahmin is nothing to me.” (Garuda Purāṇa 1.231.9)
This being said, When Scripture say Chandala devotees are worshippabe, their touch purifies Brahmanas and they have aquired merit of reading Vedas and holy piligrimages, they can enter the Temples. Thirupan Alvar a Panchama was carried inside by a Preist on his head!! Pillai Lokacharya said birth in lower caste is better for Moksha for they have humility. From "The Life of Ramanujacharya"
Ramanujacharya gave Panchamas the title of “Tirukkulattar” or the
“Blessed Descendants,” and further allowed them the privilege of
entering into the Holy Temples. Ramanuja opened the doors of religious
instructions for these classes, as well as certain modes of ritual and
other privileges peculiar to Sri-Vaishnavas, such as the
Pancha-Samskara or the Five Holy Vaishnava Sacraments. It is thus
evident how seriously had Ramanuja bethought of elevating the
low-classes as regards their eligibility for salvation. Ramanuja had already confirmed too in his own days, what his predecessors had ordained, viz.,the enshrinement of saints in the Temples, to whatever caste they may belong to.
