I agree with @Fabianhohl: the power of the Blentec800 is certainly not the reason why oat flour or rice flour cannot be produced - there are also comparison videos on exactly this question here: https://www.youtube.com/watch.

I wouldn't necessarily blend dry rice into flour. I would soak or cook it first. Dry rice is extremely hard - I would rather grind it, as it will certainly get hot in the blender due to the frictional heat. In addition, the rice grains will probably scratch the surface of the jar, which will then become dull - but functionally that doesn't matter.
But in my experience, the power of the Blendtec will not be a problem. :-)
Making oatmeal from flakes can be done in a few seconds - but it also gets warm. For making oat drinks, I always blend the oat flakes in water - it's gentler.