Can't bring up GUI while in a portal block

Pressing escape, E, or T does absolutely nothing if you're standing in a nether portal. This can cause huge problems for servers and single player worlds alike.
If you're in a multiplayer world, players can create trapped portals that are encased in obsidian. Whoever finds themselves stuck inside will have no means of communication with other players on the server. They can't kill themselves. They can't type /kill. They can't even ask for help if they can't break the blocks trapping them in.
If you're in a single player world or in a multiplayer world and you find yourself in a portal that you've completely blocked off, you can't exit the world, and the most common option for exiting the game at this point is by shutting down the computer. Even when you've done this, the problem is not fixed. Your world is effectively bricked until you figure out a way to break the obsidian.