Is it permitted to have a hull with a beam less than 100 mm?

No.

For a hull certified to the 2002 CR, if the maximum beam of the hull is less than 100 mm it will not be possible to use the depth gauge in the prescribed way to test compliance with C.5.2.

If the hull beam is 100 mm or more it will be able to meet class rule C.5.2 but this may impose restrictions on the design of the hull appendages, as on a 100 mm wide hull the gauge may touch the hull at only one section and will have to rotate about a single point of contact.

For a hull certified under the 2016 CR, D.2.4 specifiies a minimum hull beam of 100 mm.