We have a restriction in Daffodil that doesn't allow lengthKind pattern for hexBinary, but I recently reviewed the spec and can find no such restriction.
Did I miss the restriction somewhere else? I find no occurrences of 'pattern' nor 'hexbinary' in the current errata document.
I would say it is inconsistent to disallow lengthKind pattern for hexBinary, after all, we allow lengthKind 'delimited' which scans the binary bits/bytes, and that means dfdl:encoding is needed for hexBinary when lengthKind is 'delimited', so we're already allowing the other things we would need to support lengthKind 'pattern' also.