I've heard おいたん is "kiddy speak" equivalent to
Uncle in English.
Does it have any specific connotations? (Certain parents older/younger sibling?)
If used jokingly what kind of feeling would it give to the person being called?
I don't think there are any specific connotations.
It can be any uncles (either parents, can be younger sibling, can be older sibling) but I can see that some people call a middle-aged man who is not related as
If I hear/see a man being called
おいたん by a girl, I'd think that:
おいちゃんwhereas we were supposed to call him
おじさんbecause he was very kind and nice to us and we all liked him. But I think
おいたんsounds more like a
kiddy speakbecause it sounds more like a baby)
This only expresses kids that cannot speak right at the beginning of life.
They try to say おじさん, but the じ becomes い, and the さ becomes た, hence おいたん.
Semantically I think おいたん is exactly the same as おじさん, but I have never heard anyone say おいたん in the real world. In fiction, おいたん is sometimes used to exaggerate the childishness of a character. For example, according to Nicopedia, Uncle Jesse is called おいたん by Michelle in the Japanese version of Full House.
Just like English uncle, a おじさん refers to both younger and older siblings of both parents. But additionally, you can address a middle-aged male stranger using おじさん in Japanese. This is like English "buddy" or "mister".
I can't imagine how people would feel if you used おいたん "jokingly". Depending on your Japanese level, perhaps people will be simply puzzled for a while, and then try to correct the word. If you're lucky, maybe some people would ask you if you're a fan of Full House.