LED lights, make your own?

A clear plastic tube would break pretty quickly, perhaps even instantly when weight is applied - not only is this very unsafe as you would lose control of your bike, but the way plastic fails would guarantee that sharp shards would be left for you to puncture yourself as your support collapses.

I will let you visualise which part of you body gets stabbed with the shards ! :S

You are also unlikely to be able to apply enough torque at the seatpost clamp to stop the plastic (which has a low friction coefficient) from slipping - the post will likely crush first.

And you would also have to work out how to fix the seat clamp assembly to the top of the plastic tube.

This also sounds not only much much harder to do, but very dangerous and in no way robust enough to ride.

fair enough, point taken, but surely you can get seriosly thick plastic (im talking like more than 5mm thick stuff) for this.
It just seems that inserting a light inside the post is the practical way to do it.