While we do not think it's a good idea to allow additional roles / aria-* attributes on the picture element, since it is not supposed to be directly exposed to the a11y-api, there are potential use cases for setting the entire picture, and thus the graphics it represents as hidden. While this should be done with alt="" on the fallback image element, allowing aria-hidden=true on the picture itself should be allowed as this will also allow authors to hide the element.

Action item:
The aria-hidden attribute can be specified on the picture element as the only allowed aria attribute.