So, looks like things worked out quite well today.
Incredibly, I found a piece of PA6 nylon tube with exactly the correct o/d and i/d. I did as I said and drilled a 2.5mm hole in one side of the tube, and cut the other side. Slid the tube down the cable to the barrel, opened up the slit side and wrapped it round the barrel. The whole assembly fitted really snugly in the lever. Apparently, PA6 semi-rigid tube is very hard wearing, and is used in all sorts of automotive applications. When the cable arrives with the correct size barrel, I'll use my 'adapted' cable as a pattern for it and keep it as a spare. I'll also do what several people do and carry a spare cable (without the outer), thread it through with a screw clamp nipple as a 'get me home' fix. Next job will be to see how my soldering skills are!
I also had a problem with my bar-end mirror - the metal expanding thingumajig fell apart, so thought about how to fix it. The solution turned out to be a length of old car radiator hose, with a coned cup and a nut on the end. Had to bash the whole assembly in with a bit of washing up liquid, but it's tightened up and doesn't move (I reckon rubber has better grip than metal anyway).
Always nice when a plan comes together
