Good morning Retro:
Metta Sutta AN 4.126
There is the case where an individual keeps pervading the first direction — as well as the second direction, the third, & the fourth — with an awareness imbued with good will. Thus he keeps pervading above, below, & all around, everywhere & in every respect the all-encompassing cosmos with an awareness imbued with good will: abundant, expansive, immeasurable, free from hostility, free from ill will. He regards whatever phenomena there that are connected with form, feeling, perception, fabrications, & consciousness, as inconstant, stressful, a disease, a cancer, an arrow, painful, an affliction, alien, a disintegration, an emptiness, not-self. At the break-up of the body, after death, he reappears in conjunction with the devas of the Pure Abodes. This rebirth is not in common with run-of-the-mill people.
A related sutta is
Metta Sutta 4.125:
There is the case where an individual keeps pervading the first direction[1] — as well as the second direction, the third, & the fourth — with an awareness imbued with good will. Thus he keeps pervading above, below, & all around, everywhere & in every respect the all-encompassing cosmos with an awareness imbued with good will: abundant, expansive, immeasurable, free from hostility, free from ill will. He savors that, longs for that, finds satisfaction through that. Staying there — fixed on that, dwelling there often, not falling away from that — then when he dies he reappears in conjunction with the devas of Brahma's retinue. The devas of Brahma's retinue, monks, have a life-span of an eon. A run-of-the-mill person having stayed there, having used up all the life-span of those devas, goes to hell, to the animal womb, to the state of the hungry shades. But a disciple of the Blessed One, having stayed there, having used up all the life-span of those devas, is unbound right in that state of being. This, monks, is the difference, this the distinction, this the distinguishing factor, between an educated disciple of the noble ones and an uneducated run-of-the-mill person, when there is a destination, a reappearing.
The first passage seems to imply the practice of insight + metta leading to the Pure Abodes, the second passage the practice of metta leading to rebirth in the Brahma world and from there being Unbound.
I remember the passage (passages?) you're talking about where metta +insight can take you "all the way" in this life, so to speak, but I can't find it right now!
EDIT: Another relevant sutta with notes at the bottom that link to various other suttas about metta:
Metta Sutta SN 46.54
retrofuturist wrote:Greetings,
There is a specific sutta I am seeking, and I hope someone may recall it and be able to share it with us.
The sutta starts with a standard metta cultivation, but as the sutta progresses, the focus switches to observance of the emptiness or impermanence of the sankharas created (through the metta cultivation?). The sutta may (but I am not 100% sure) regard such things as a dart, alien etc. to be removed.
Either way, the insight aspect of the cultivation is then pursued through to arahantship.
The sutta is unique in the sense that it shows of a metta practice which is fully liberative, not merely one that takes the yogi up to the fourth jhana.
Does anyone recall this sutta or have a link or reference number for it? I have some vague recollection that it may be from the Anguttara Nikaya but I may just be making that up.
(I recall the sutta being stored at the original Dharma Paths forum, but alas, it is no more. I regret that I have been neglectful in not referencing/storing it properly. Any help anyone can provide to help re-find the sutta would be greatly appreciated...)
Metta,
Paul.

"People often get too quick to say 'there's no self. There's no self...no self...no self.' There is self, there is focal point, its not yours. That's what not self is."
Ninoslav Ñāṇamoli
Senses and the Thought-1, 42:53
"Those who create constructs about the Buddha,
Who is beyond construction and without exhaustion,
Are thereby damaged by their constructs;
They fail to see the Thus-Gone.
That which is the nature of the Thus-Gone
Is also the nature of this world.
There is no nature of the Thus-Gone.
There is no nature of the world."
Nagarjuna
MMK XXII.15-16