Which non-rice grains cook up most like sticky rice?
From a purely sticking perspective, I would say golden millet. When I leave a cooked batch in the fridge, it goes to a single slab within a few hours. However, tastewise, it is much more reminiscent of wheat than of rice. It is very mealy in texture, too.
Alternatively, you might want to experiment with adding tiny amounts of starch to a different grain, to make it stick, or maybe using pap instead of rice, possibly embedding some other grains (or maybe something like tapioca pearls) to have some variety in the texture.