Apply a subtitles file to a rented movie in iTunes
iTunes Videos are DRM-Protected and are not saved entirely on the local Drive AFAIK.

Therefore it is very hard to kind of "inject" a subtitle file.

I think Apple made this restriction due to quality reasons. They want a product to be very clean and complete as they can officially provide it.

Some Movies don't provide the Info officially and some other movies simply are'nt worth it..

So no,there is no official way. I guess you can only give Feedback to Apple, describing your problem and hoping to fix this in the future.

BUT
: There's always an App for that !