Well, we could always ban vanishing dye, but that doesnt solve jewelery swaping or swaping items with identical look.
How about something along the lines of
"Players are not allowed to carry any items in their inventory except for healing potions* (and xy if you decide to allow certain items). Once per duel, a player can request his opponent to record and upload a video of his inventory. A request is only allowed if both players are in the arena and the round hasn't started (e.g. instead of "go"-"go" the round would go "go"-"show inventory please"). Upon request, players are not allowed to leave the arena until the video was uploaded and reviewed."
This would require the possibility to upload short videos for your challenges on d3cl. All players would need a free recording tool like Bandicam or even just a mobile phone/camera. They wouldn't have to run it all the time, just for the short video upon request.
What do you think of this? It should be an effective way to prevent item-switching. I also believe that players wouldn't make use of this rule too often, so the time delay should be minimal.
*maybe the inventory should be filled with white/blue items to prevent players from dropping/hiding items for the video.