If there's a skin that isn't on live servers yet, and I have several unopened chests, is it possible for me to unbox that skin once it releases to live, using boxes that I obtained before the skin release?
I can only offer personal experience, but my experience has been that it is determined when the box is opened.
I use to stockpile a TON of boxes and chests, not opening them for many months at a time. When Gwen was released, I opened all of my boxes and was given a Gwen Champion Shard. I do not believe I had obtained any new boxes since her release.
Because of that, I have the assumption that it is determined on opening, but I cannot say for certain as I did not record specific information. However, that does at least give me a very high certainty.
Normally I would not post without more specific information, but considering the age of the question and the lack of other responses, I am making an exception to a least share my experience, and what I personally consider to be a proven theory.