There is no awareness beyond the khandhas.
There is no such thing as the "Thai Forest Tradition" having a uniform set of teachings. The "Thai Forest Tradition" is just a variety of different jungle gurus who made up their own versions of Buddhism.
Ajahn Amaro has wrong view when he said: "that which knows the khandhas is not part of the khandhas".
SN 22.48 refers to two types of five khandhas. Five khandhas without clinging and five khandhas with clinging.
SN 22.85 describes the end of the life of a Buddha as the ending of the five khandhas. Therefore, there are pure undefiled liberated khandhas.
SN22.85:13.1: If, friend Yamaka, they were to ask you: ‘Friend Yamaka, when a bhikkhu is an arahant, one whose taints are destroyed, what happens to him with the breakup of the body, after death?’—being asked thus, what would you answer?” “If they were to ask me this, friend, I would answer thus: ‘Friends, form is impermanent; what is impermanent is suffering; what is suffering has ceased and passed away. Feeling … Perception … Volitional formations … Consciousness is impermanent; what is impermanent is suffering; what is suffering has ceased and passed away.’ Being asked thus, friend, I would answer in such a way.” “Good, good, friend Yamaka!
"Lokuttara" means "beyond the world"; which means the mind not attached to the khandhas. SN 20.7 says "lokuttara" is "sunnata" ("emptiness"), which means "empty of self & anything pertaining/belonging to self". It is the mind that does not regard aggregates as "self". "The mind" is also a khandha. A khandha does not attach to the khandhas.
For example, the Buddha called liberation "cetovimutti" or "cittavimutti" (MN 30). Or Dhammapada 154 says the mind "citta" of the Buddha attained the unconditioned. "Citta" ("mind-heart") originates from nama-rupa (SN 47.42) therefore is part of the khandhas.
Or MN 43 says wisdom & consciousness are cojoined. Wisdom is required for liberation. SN 22.53 describes liberation in terms of freed consciousness. Consciousness is a khandha.
The Buddha never said liberation is possible because of the transcendent quality of awareness beyond the Five Skandhas. The Buddha did not even use the word "awareness", which is too broad or vague to correlate with a specific Pali world.
The Buddha said liberation was possible because the element of Nibbana exists (Ud 8.3). Nibbana is not the khandhas but is a sense object (ayatana; Ud 8.1) known by the khandhas.