If there are, I haven't found any yet and I do wide searches for stories all the time. I think you're right that there is no demand for them.

Dr. Hypothetical, Jonathan's old buddy from the Army.
Judge Hypothetical, Martha's first cousin on her mother's side.
Nurse Hypothetical, Martha's friend from childhood? Or sister? Jonathan's sister?
Unfortunately there is no basis in canon for this. Phooey.

Well someone had to be there to provide medical records or he would have never been to school. Even if he traveled the world under his own power he would need a stamped passport to get a job at a newspaper or sell stories to a newspaper in one of the countries he visited. He needed a birth certificate for that, which he would have received after the adoption. (I wonder why his old suitcase didn't have stickers on it from around the world?)
The theory does stand to reason.
We'll never know who they were but that small core in on the secret were there- in recent Superman history anyway.
Back in the 30s the months long snowstorm and Martha giving birth on the farm would have worked I'd guess. It was the dust bowl era and the weather must have been affected by the drought. Of course the unwanted child of a relative would have been more believable.

Kate, wondering if Jonathan would have been in the army with Dr. Hypothetical before or during Elvis' time

We actually talked about it once in the RPG when we made up a silly Stripes-like situation between Jonathan and Elvis, but it didn't work. (Remember that movie?) We figured the very youngest Jonathan could be at the beginning of the series was 60 in 1993,and 20 in 1953. If he joined up at 18 for four years, he would have been halfway through his enlistment by then. Elvis enlisted in 1958 for 2 years.
No reenlistment for Jonathan. Out in 1955, he and Martha find Clark in 1966- giving them about a decade or less of marriage.

Very creative writing might get it to work out. We had Tempus trying to get Elvis to kill Jonathan in Germany! Let your imagination go from there. LOL!

My imagination doesn't go anywhere close to where you and your RPG people go. (Thinking of Glock and Numerman.)
So Jonathan and Dr. Hypothetical probably would have served in the Korean War by that timeline. A wartime friendship would be why the doctor was willing to help his friend with the baby he found and the doctor might know more about medical experiments going on around the world to have assumed this is what might be the answer.
No wonder writers have such detailed backgrounds for their characters. It's fun making up stuff! No Elvis but this is better.

Kate

Good start. Two guys from Kansas going through war, coming home and staying in touch, maybe living not too far from one another. The friend goes to college under the GI Bill and becomes Dr. Hypothetical; Jonathan could use it for loans to build up the farm instead of college, or maybe classes at a community college where he meets Martha. (?)
The farm is prosperous enough for Jonathan to be able to send Clark money from time to time without a second thought.
What do we know about the Kents aside from five tries before Martha married Jonathan?

Nothing I can think of beyond that.
Jonathan has a plow that can turn over frozen ground? Superplow!
What about grandparents? Cousins? If Clark had cousins he wouldn't be the lonely, friendless, bullied kid from fanfic and the first Reeve movie, sometimes hated and feared. As nice as it is having the close relationship between him and his parents, he must have had a best friend, an ally and confidant. He knows how to make friends. Someone who grew up with him knows.
We don't see them in canon but they must have been there. Not in the movies, obviously, otherwise he would have known how to fight and land a punch or two in his own defense!

Kate

In Smallville, Martha's father was alive but since he didn't get along with Jonathan, he refused to have a relationship with Clark, though Clark wanted to know him better.
Oh, no comment on Reeve Superman getting beat up, but he had superpowers right from the beginning. TNAOS got them gradually so he had time to form friendships. Even 'Smallville's' Clark did, not to mention comics Clark. Even if there was only one cousin he was close to, that cousin would know and protect him.

I'm not a comic book reader nor a Smallville watcher, but I do know about Pete. What about Pete? I think he found out about Clark in Smallville. Does he know about Clark's powers from childhood in the comics?

In the Superboy comics Pete and Clark are best friends. Pete knows Clark is Superboy but never tells him he does. He helps him much like Lois did on the show. He makes up excuses for Clark's absences.
That was the Silver Age of the comics, though. Everything changed and Superboy doesn't exist in the comics anymore. Clark no longer has a history of Superboy in his life. They did away with Superboy completely from what is now Superman's life story. They decided he existed in I think they called it a pocket universe, or something like that then they destroyed that offshoot universe.
Like with Batman, Superman's friendship with Pete has changed. He and Batman are now rivals and he and Pete are two guys who know each other from school. But then he was killed at one time by the Prankster and Toyman (?) who tortured him until he told them Clark was Superman but he showed up recently as vice-president to Lex Luthor's president of the US.
In some appearances Pete's married to Lana Lang and is jealous of her friendship with Clark- which is close because she knows Clark's secret and Pete doesn't.
I didn't watch Smallville either but I think he did know Clark's secret.
He was never mentioned in TNAOS but he could very well be one of the people who know and help Clark behind the scenes. Jonathan and Martha did wonderfully raising Clark but someone else taught him to be relaxed with people of his own age. Pete, a close cousin, someone was there. The Fortress of Solitude had room for more than one boy.

Kate

Wow. Talk about brutal. In the comics the Kents are much older than the Kents from either TV show and they were more elderly in the Kirk Alyn serials, but in both TV shows Clark could have known grandparents from one or both of his parents as they are portrayed as younger than the comics.
The elder Kents of TNAOS could have retired when Jonathan took over running the farm. Martha's parents would have been happy to see her with a child and have wanted know to Clark.

Kate, whose own grandpa was in the navy and loved showing us pictures of where he'd been and who wished he could have retired to Panama but didn't