Is it within the rules to have the sole sprite be a live2D character?
(It will have expressions and idle breathing, but the pose will not change)
Eyes moving is fine, head movement would also be allowed, if there were no new frames added to create the animation, meaning if you're working just with idle skeletal animation, a head bobbing is fine. Turning the whole head to the side etc would be allowed only as a variation on a single body part (i.e. no frames in between). It would also mean you can't have an extra object you can hide/show (hat, mask), you'd have to choose one or the other - one change of body part, or extra object.