I don’t know why we’re engaging in this conversation, but since I read your comment it made me solve the issue from a mechanical standpoint.

Have the buttplug be a sort of reverse umbrella, so that at first it’s a normal buttplug, but like you twist it or pull a string or something and the part in your rectum actually opens up, pushing something soft against the wall of the rectum and spreading the base just a little bit to get an opening large enough to produce discernible shapes.

Might be really weird though, because as you say, it’s kinda against nature, so this should mean that your sphincter closing in on the buttplug wouldn’t stop shit. ^Ba-dum-tss