The rule you're referring to applies to sanctioned events exclusively, as indicated for example by Spawnsire of Ulamog's Rulings:
In a casual game, the cards you cast from outside the game come from your personal collection. In a sanctioned event, those cards must come from your sideboard. (...)
From what I can gather from the official product page, there will be sanctioned drafts, which means the above rule will indeed prevent you from searching anything else but the cards in your Sideboard (meaning that you drafted, but didn't include in your deck), and as you correctly stated, there won't likely be any banned or restricted cards to find there, as the set itself is legal during it's own draft.
As you said yourself, this sounds like a case that will be handled by the Release Notes, which are yet to be publicised. Seeing as Un-cards are meant to be exciting and bending the rules as far as possible, it seems to be in the spirit of the set to ignore this restriction and actually be able to search for your Time Walk if you choose to do so. After all, with a set like this, there will be crazier things happening, and a 1/1 with CMC 4 isn't quite worth the pick otherwise.
Whether the release notes will account for that case or not, you could address the head judge and ask them to allow cards outside of the sideboard as well, as they have the authority to rule one way or another if in doubt. While this would be questionable in any other tournament if it went explicitly against what the release notes say, I personally wouldn't mind for a set like this, and I think most players partaking in tournaments like that would expect a less serious athmosphere from the beginning, and/or are enjoying more crazyness.