One thing to consider with those bar ends that Chadster linked to: it looks like those are clamped to the center ferrule on road bars, with a diameter of 31.8 mm. But if you're clamping to the handgrip area on trekking bars, the diameter is probably 22.2 mm. Which means that without a massive shim, they won't work, and even then, might be unsatisfactory. That is an interesting find though.

Also: it's possible that even if you took off the grips on your bars, the clamps on normal bar-ends wouldn't be able to make it around the bends. But I'm not clear where exactly you want the bar-ends installed, or what hand position you're trying to achieve. There are some interesting alternative handlebar designs out there now that might do what you want.