There would be a [very] small impact on gameplay if the hookshot was swapped with the boomerang. You don't have to redesign the overworld to accommodate the hookshot and doing so wasn't part of the hypothetical situation I asked you about. The only change is the item swap. There are no boomerang triggers in Z1 and it isn't required to defeat any enemies. The Nth quests have continued in the tradition of 1st and 2nd but have also become more lenient to fit in with more modern ZC quests. Letting 5th quest entrants have the ability to alter or 86 staple Z1 items that have little effect on gameplay is just fine in my book. The boomerang, red candle, and wand only serve to make the game accessible to beginners and people with little skill without being overly difficult. This is why many skilled players consider these items to be almost game-breaking.
That is a decision they will have to make isn't it? You can make strict rules about items but it will only discourage possible participants or have them looking for loopholes. Fun in your book might mean "too easy" but some people like a fair challenge and the absence of overpowered items.