In his 1985 short story "Under Siege," Martin tells the story of a futuristic mutant assigned to warp back into the mind of a Swedish soldier in order to influence the outcome of the Siege of Sveaborg and, in broader terms, the Finnish War. While a machine's involved, there's no flying TARDIS or electrified DeLorean popping out into the 19th century. The highly evolved man plugs in straight to the mind of his past proxy (similar to Assassin's Creed's time travel logic) and influences action like a conscience. The voice in the back of your head? Could be a time traveler.
There are risks involved. As one of the future engineers explains, “The longer you stay in rapport, the stronger your influence grows on the host, and the more likely it becomes that your presence will be felt. Few hosts can deal with that perception.” The line echoes the Three-Eyed Raven's warning to Bran in "Oathbreaker." "Stay too long where you don't belong, and you'll never return." Ol' Three-Eyes insists that his student can't influence the past ("The ink is dry"). Still, he urges Bran to stop reaching out to his father. Perhaps because he did the same thing when he was a young man? Something forced Three-Eyed Raven to live inside the Weirwood for 1,000 years, "watching the world from a distance."