This could be implemented in-game or on the spiral knights site or wiki. This will allow you to find nice combinations for a costume armour you may want to work towards. It can help you find an ideal look for your knight for the future. I guess we can partly do this with the AH but we first have to search for it, then click on it, then click preview and each time it will bring up your knight and when your done you have to press close and you have to go through this for each piece of equipment. You also won't be able to preview it if no one is selling it or if it's not sellable (e.g. heavenly guardian armour).

With this you could have a basic knight on one side and a search bar on the other, and all you would have to do is click once on the item you wish to preview. When searching for one item, it could show you other items in the set if it is part of a set (e.g. snarblox set with the coat, cap, shield, blade, bomb) in another section underneath what you searched.