Rob I'm a little confused and maybe you can clarify this for me. I'm not an instructor and please don't take this as me being smart. I had very mentor-based instruction in technical and cave diving, so he dove me "up a level" regularly as we progressed. Seems to me that it isn't so much "written" as it is "expected" that when you're learning cavern, you dive in caverns, and when you learn trimix, you dive trimix, even though you're not certified to dive those levels yet.
Obviously I wouldn't condone an instructor randomly taking his buddy on a trust-me dive, but I see that as different than a cave instructor taking a student who just completed cavern on a little bit of an intro dive to give him a taste of it and start him down the path to completing that course?