Is it acceptable to post questions to which we already know the answer if it is a particularly interesting and obscure science fictional factoid? For instance today I found out that the origin of the term credit card came from a seminal work of science fiction.
There's no rule against posting a question you know the answer to. (Hey, how are we to know?) And this is a good question -- it's also not something that shows up as soon as you Google for it.
Please do it only if the question and answer are truly interesting. If the answer took some research on your part that you're now sharing, fine. I'm not convinced that the etymology of a word fits (or even that it's on-topic).
Some people (including prominent developers, but I can't find links to the posts I'm sure I've read about this...) use StackOverflow much like this - where they once would have written a blog post outlining a problem they had and how they found the solution, now they write the same information into a SO question and answer.
Although it's allowed, I think in the vast majority of cases it works better to not do this (except on meta). If there's something sci-fi/fantasy related that I really want to know, enough that I'm going to do my own research into finding it out, then I think the first thing I do is write out the question on the site. While I'm trying to find the answer myself, other people might help me out, and if they don't then I can hopefully answer it myself.
I think this is more efficient, and it avoids appearing like you're just trying to gain rep. The end result (a high-quality, interesting, question and answer on the site) is the same.
If it's something that doesn't actually need any research - e.g. the "factoid" or "trivia" mentioned, then I don't think the problem there is that you know the answer, it's that the question/answer isn't substantial enough to be interesting.
For your specific case, I'm not sure how you'd ask that question without knowing the answer (i.e. if you don't know that the origin of "credit card" relates to a sci-fi story, why would you ask on this site?). I would find it more interesting if it was something like:
The term "credit card" was first used in 1887 by Edward Bellamy in Looking Backward, while the first bank-issued credit cards arrived in 1946, and did not use that term (the "Charge-It" program from the Flatbush National Bank of Brooklyn, NY). Did the convergence on "credit card" as the name for these items arise from Bellamy's work, or was the name independency invented and popularised by someone else (it's a "card" that you use for "credit"!)?
(This is tenuously sci-fi, given that it's asking about the influence a utopian book had).
This includes the "factoid", but expands it out into a larger question, to which I don't have the answer.