Yeah, I think that at the very least the prestige skins should always be available from the start since they require a lot more planning to decide if it's worth investing time to try to earn them - if a player doesn't see the event guide, at least it's been visible in the client from the beginning of the event. The minor things aren't as important, but it would still be more convenient to see it there.
A good compromise would be to simply have a placeholder in the crafting list for loot that hasn't come out yet; hovering over it says something like 'Will be released soon!' or something along with the token cost. The event guides are nice and cover Riot's butt for complaints like the one the OP is making, but that doesn't mean there aren't ways to improve things.
Maybe even include a link in the client to the event page itself. I just checked right now and I don't think there's a link to the PROJECT event page - I'm not sure if they included something like that for the previous events either.