Unless all our sources are wrong, impossible. There is absolutely no evidence for this that is not either made up or incredibly circumstantial. The story is only pushed in fiction and by conspiracy theory authors (just like the story that Uesugi Kenshin's a girl). Even the self-proclaimed Mitsuhide's descendent argues against it. And he goes around trying to convince people that it was Tokugawa Ieyasu who plotted to kill Nobunaga.

Not only are there no evidence for the story, there's two strong evidence against:

1. Tenkai took up position at a monastery in Ieyasu's realm and began advising him in 1588, a decade before Hideyoshi's death. Hideyoshi's legitimacy was in many ways derived from avenging Nobunaga, and he had as his vassals many surviving members of the Oda Clan. Mitsuhide was Nobunaga's right-hand-man when he was alive, meaning many people knew what he looks like. Imagine the shit Ieyasu would be in if someone told Hideyoshi that Ieyasu was hiding Mitsuhide. Why would Ieyasu take this huge risk for miniscule (at the time, effectively zero) gain?
2. We know when Tenkai died. While sources offer different years for Mitsuhide's birth, if Tenkai was Mitsuhide then he lived between 110 and 130 years.