I had never seen anybody do this or come up with a trick for it, but I figured that if Dungeon Key Doors were possible, Big Treasure Chests should be too. This is what I came up with.
Before Key
Kill the Keese
Get Key
Open Chest
It works exactly like a Boss Key Door when setting up the Boss Key. Then make the room with the closed Chest and put the fire enemy in the corner, and set it up with Boss Flag corresponding to Boss Key room. The set up pit combos that look like the floor as secret combos (Enemies -> Secret Flag, too) on a square they would the player have to step on to get to the chest. Copy the Closed Chest room to another screen, minus pit combos and fire enemy. Replaced the closed chest with an open chest tile and place the item in the open chest. Make sure you make part of the chest walkable, so you can get the item. Then set a scrolling side warp out of the room, to the room you would have went to, if you had left the Closed Chest room
If someone has already figured this out and posted, I apologize. But I had never seen anything about it.