So I used console commands in Oblivion to spawn in the Emperor's Helmet, but it isn't showing up in my inventory. It has the weight added but just not the icon to equip it.
If the item you added in the console can not be seen in the inventory, that means that the item was not meant to be used by the player (as answered by @z '). However, you can use the
player.equipitem <form ID> console command to force equip it onto your character. (The form ID of the Emperor's Helmet is 1FED2.)
Here's a screenshot of my character with the Emperor's Helmet equipped. Notice that my inventory doesn't show a head apparel equipped nor is the Emperor's Helmet in there:
If you're willing to use the Construction Set, you can mod items to be player usable (allowing them to shown and be equipped in your inventory) by locating them in the CS and checking the "playable" box.1