Daffodil doesn't allow lengthKind 'pattern' for hexBinary, but there is no such restriction in the DFDL specification.

The workaround is to use type xs:string with dfdl:encoding of 'iso-8859-1', but this is less attractive since the strings will be full of non-printing characters typically, in particular lots of zeros, which will get remapped to &E000; Private-Use-Area characters when output as part of the infoset. In general it will be HARD for anyone to consume such an infoset element - they would have to implement the inversion of PUA characters back to the character code bytes that are not allowed in XML.