It should be fine to transmute from NonZeroU8 to u8. To allow this, I don't think sound transmute should require commutative and instead of requiring the same set of niches , between T and U it should that the set of niches of T is a super-set of the set of niches of U. This would allow transmutes that strictly increase the amount of values a type can have, like NonZeroU8 to u8.
Note that transmuting between NonZeroU8 to u8 is similar to transmuting from &T to *const T.