There's been a lot of discussion over at Queerly Gen about what constitutes gen; the consensus seems to be that if the focus is not romance/sex, it's gen. For me, it's not gen if it has non-canonical romances, queer or otherwise, but that definition got shouted down as too restrictive. I'd call it slash/gen, het/gen or femslash/gen if it has a non-canonical background relationship. It's partially because it's so easy to write an apparently gen story that's actually all about a pairing - not that your story sounds at all like it would be.

:D I think you should write it. Time-travel, however redundant, happens all the time in all the Star Treks. The first time it happened it wasn't even discussed, it's like "So, we were monitoring 20th century Earth when..."