2406.1 Human impact loads. Individual glazed areas,
including glass mirrors, in hazardous locations as defined in
Section 2406.4 shall comply with Sections 2406.1.1 through
2406.1.4.
Exception: Mirrors and other glass panels mounted or
hung on a surface that provides a continuous backing support.
I would accept a solid surface backing that won't bend. Careful, most new medicine cabinets mirrors do not have a backing.