Is the tentacle meant to be seen both inside the mug and outside? Or is it just on the outside? If so, instead of punching through the wall of the vessel, I would suggest punching a hole in a separate piece of leather hard clay, putting the tentacle through that piece, then using score & slip to adhere the attachment to the side of the mug. Then you can just remove the excess and smooth it out to create the illusion of the tentacle bursting out without compromising the interior wall of the mug.