Use the /sneak command to appear sneaking.
While sneaking, your nameplate cannot be seen through walls, your movement is unhindered, and you appear crouched. In order to stop sneaking, you can either use the /sneak command again, or press the shift key to actually crouch.
I am willing to add addition functionality as requested.
- ToggleSneak.sneak - Allows you to use the /sneak command
- ToggleSneak.sprint (default OP) - Allows you to sprint while sneaking. (Otherwise you stop sneaking upon sprinting.)