Thus I resolved to build a handle-bar-mounted monitor using fibre optic cable. It's taken me a couple of goes to get right, so here's the info.
First I just tried taking the bare fibre out of its sheath and threading it along protected bits of the frame and gaffer-taping it on. This set-up had two significant problems. 1) one fibre is too dingy - you can barely see it, and 2) It lasted one trip of about a mile and a half to the pub before the fibre was broken.
So I did it again a bit better this time. The cable I have is two fibres in a loose plastic sheath about the diameter of brake cable. I took the fibres out of another length and threaded them down the cable alongside the two already there to get a cable with 4 fibres in it. This is about the minimum brightness you would want to use (depending how good your rear light is I suppose!). The result is easily visible on dark roads, but is difficult to see under good streetlighting.
The ends of the cable are positioned using stiff garden wire to which the cable is tie-wrapped. At the rear light the cable is positioned a few mm from the side of the rear cover. This means there is no physical attachment to the light so it is still easy to change bulbs or to remove. At the front I first had it just coming up the stem, but found that you actually have to look down to see a monitor there - it's best if it is on the handlebar. I have it just peeking out in front of the bar. As this is an exposed bit of stiff wire sticking straight up at you, I arranged it so that it is shielded by the bar so that if you crashed it wouldn't impale you. It is easily visible but not bright enough to be distracting. The length of the cable is gaffered or tie-wrapped into place in traditional fettlers style.
I've found it very handy and will probably do it to some other bikes. Next time I might try 6 fibres for better in-town visibility.