I can't see why you could not build a small resonator, it may have an interesting sound because of the higher pitch.
Maybe to get started you could look up how to make a tin can resonator, and make one out of a small tin or a sawn off coke can. If that works and sounds reasonable, the next step would be to fit the sawn off can or a small pie dish into a tiny uke and get it to work. I think it might be a useful learning exercise, but perhaps not commercially useful, unless you can get an amazing sound.
The people at Cosmos are mostly grumpy old men, but they have some good points to make about banjo ukes, particularly when the BU is played poorly in public. The high BU notes travel well and the bass doesn't, leading to an ugly annoying sound at a distance in a crowded place that is not a performance space. Resonating ukes are also loud, makers of the better instruments do a bit of work in the design and construction to tame the rogue resonances and set up a pleasing sound. In a tiny uke, once you get the reso working, then you will probably need to look at how to do the design and construction to refine the sound, which I expect will be high pitched.
