LED lights, make your own?

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.

You need to find a plastic tube that is thick enough to support your weight, not crush under the torque required to secure it in the frame with the seat post clamp. You will also have to make sure it is of the right diameter to get into your seat tube, make sure it is hollow and either transparent or at least translucent. I doubt you could find something to meet these specifications exactly, but I suppose you could get something custom made.

It will also need to be stiff enough so that it does not bend when weight applied - not particulaly to prevent a bendy seatpost but to avoid repeated stress to the structure - and it would also have to be pliable (non-brittle) enough to allow a little flex so that repeated application of weight doesn't fatigue the structure of the plastic. Also nicks or scratches in plastic in a stressed situation will likely lead to stress risers (which encourage fissures usually leading to failure).

No one, as far as I know, has ever made a plastic seatpost, even on the very cheapest £49.95 Halford's death traps.

You will also have to work out a way to safely secure the seat clamp assembly to the top of the tube.

I can't see how this is 'way easier' than wrapping some electroluminescent film around the seatpost you already have ?