The character of Mestral is a key player in the Trek novel From History's Shadow. Mestral is brought forward 300 years into the future where he reveals that he has been living on Earth since the 1960's and has been instrumental in a number of key scientific and cultural events. The very clear implication is that his continued presence on Earth is vital to the ultimate foundation of the Federation:
“I am grateful to you for allowing me to return to Earth, Miss
Lincoln.” He nodded to Kirk. “And to you, Captain.”. “Earth?” McCoy
asked, frowning. “Not Vulcan?”
Mestral replied, “It is my desire to continue my observations of Earth
and humanity, Doctor. The time period I left was something of a
turning point in your history, and I wish to be on hand to see what
“What if you’re discovered?” McCoy asked. “Won’t that affect our
history, too?” Lincoln said, “We’ll be keeping tabs on Mister Mestral
as well, Doctor. Besides, how do you know his being on Earth doesn’t
prove beneficial to our history in some way?” She said nothing else,
leaving the cryptic question to hang in the air as she retrieved her
servo from a pocket. She keyed the device, and a blue-black fog
appeared at the rear of the transporter chamber.
There's also short story titled "Mestral" (by Ben Guilfoy) in the fan-fiction anthology Strange New Worlds IX.
The story tells of Vulcan explorer Mestral interacting with a crazy young scientist named Zefram Cochrane, helping him navigate a world where Eastern Coalition troops are invading the United States. In addition to inspiring him to continue with his research, he also introduces his new friend "zee" to a young aeronautical engineer called Lily Sloane.
At the end of the story...
we see Mestral wandering off into the woods with an apparent gunshot wound to the abdomen. In the absence of medical assistance, his chances of survival seem slim.