You can use clear BT for anything you want. There are no NAR rules against it (or any other organization, as far as I know).
The old Estes Astron Streak was a 2.75 inch long piece of BT-20 equivalent clear plastic tube. The tubing material was quite flexible but was effectively held rigidly in the correct position when a motor was inserted. You had to use peel-n-stick paper to wrap the tube so you could glue fins on the back and the NC in the front.
The biggest problem with using plastic tubes is finding something strong enough not to collapse during thrust, but not so thick-walled and heavy that performance suffers. There are further problems with trying to use clear plastic, especially around the front of the motor mount where hot ejection gasses will fog and scorch the inside of the BT. I like to use a paper liner starting at the front of the motor mount and extending about half-way to the nose----I can just throw it away after a flight and the plastic tube still looks good. Or, if you have a plentiful supply of plastic tube, you could cut a sleeve from a piece of scrap and slip it inside the BT to the same location.
Other difficulties with plastic BT include finding the right adhesives to attach fins, lugs, etc, without melting/eating the thin-walled BT. If you find a glue that works, then you have to worry about your craftsmanship, because the glue joints around the fins are really going to show.
Anchor points for the recovery system can be either internal (at the motor mount) or external (at the base of the fins).
It would be kind of cool to find a source of matching clear plastic nose cones but I am not holding my breath.
And the old Estes Phantom display model? It was completely flight-worthy (I saw one flown many times).
