If remembering correctly I once heard a Dhamma talk about the Buddha finding solace in/with the investigation of energy while (at least once) experiencing physical pain. What's the sutta reference(s) for this please?
It's not investigation of energy, it's animitta samadhi that can relieve severe physical pain. SN 47.9:
Etarahi kho panāhaṃ, ānanda, jiṇṇo vuddho mahallako addhagato vayoanuppatto. I’m now old, elderly and senior. I’m advanced in years and have reached the final stage of life. Āsītiko me vayo vattati. I’m currently eighty years old. Seyyathāpi, ānanda, jajjarasakaṭaṃ veḷamissakena yāpeti; Just as a decrepit cart keeps going by relying on straps, evameva kho, ānanda, veḷamissakena maññe tathāgatassa kāyo yāpeti. in the same way, the Realized One’s body keeps going by relying on straps, or so you’d think. Yasmiṃ, ānanda, samaye tathāgato sabbanimittānaṃ amanasikārā ekaccānaṃ vedanānaṃ nirodhā animittaṃ cetosamādhiṃ upasampajja viharati, phāsutaro, ānanda, tasmiṃ samaye tathāgatassa kāyo hoti. Sometimes the Realized One, not focusing on any signs, and with the cessation of certain feelings, enters and remains in the signless undistractible-lucidity of the heart. Only then does the Realized One’s body become more comfortable. Tasmātihānanda, attadīpā viharatha attasaraṇā anaññasaraṇā, dhammadīpā dhammasaraṇā anaññasaraṇā. So Ānanda, be your own island, your own refuge, with no other refuge. Let the teaching be your island and your refuge, with no other refuge.
link to full SN 47.9 sutta https://lucid24.org/sn/sn47/sn47-v01/index.html#s9
========================================== Bojjhanga "parittas", as they are usually presented, or implied, do not work as advertised. There's no way you can magically say 7 words and expect to be cured of fatal illnesses or debilitating physical pain.
I believe this might be the sutta in question. There may be other similar suttas.
On one occasion the Blessed One was dwelling at Rajagaha in the Bamboo Grove, the Squirrel Sanctuary. Now on that occasion the Blessed One was sick, afflicted, gravely ill. Then the Venerable Mahacunda approached the Blessed One, paid homage to him, and sat down to one side. The Blessed One then said to the Venerable Mahacunda:
“Recite the factors of enlightenment, Cunda.”
“These seven factors of enlightenment, venerable sir, have been rightly expounded by the Blessed One; when developed and cultivated, they lead to direct knowledge, to enlightenment, to Nibbāna. What seven? The enlightenment factor of mindfulness has been rightly expounded by the Blessed One; when developed and cultivated, it leads to direct knowledge, to enlightenment, to Nibbāna…. The enlightenment factor of equanimity has been rightly expounded by the Blessed One; when developed and cultivated, it leads to direct knowledge, to enlightenment, to Nibbāna. These seven factors of enlightenment, venerable sir, have been rightly expounded by the Blessed One; when developed and cultivated, they lead to direct knowledge, to enlightenment, to Nibbāna.”
“Surely, Cunda, they are factors of enlightenment! Surely, Cunda, they are factors of enlightenment!”
This is what the Venerable Mahacunda said. The Teacher approved. And the Blessed One recovered from that illness. In such a way the Blessed One was cured of his illness.