I have several sugjestion, not all of wich I'm certain will work out. The hamer dulcimer may be an option since it just requires being able to grip the hammers. I'm not sure how quickly the hands would tire. also slide guitar could work. While a wistle with a slide in it may seam like a child's toy, there's no reason it couldn't do everything that a Irish tin wistle is used for given enough practice. Finally, the people I bought my bambo flute from sell what they call a dragon wistle. It's just a short piece of bambo that can be blown like a flute and modulated by covering the end or incerting a finger.

I hope some of this will help you.