I've used bar tape on a MTB's flat bars with bar ends. It worked very well for me! It was regular tape, not too fat/squishy, and the result felt firm but not uncomfortably hard.

I *did* twine both ends of each segment, as I had the tape loosen and unroll itself once on one side of the bars. I guess you can use twine or electrical tape depending on your aesthetics.

(I've read about people installing Ergon grips with integrated bar ends on their Bromptons and being very happy with them... definitely worth a try.)