This feature, the 'I' for ISO specifier, is not implemented in the ICU library we use for the other aspects of calendar patterns.
Before we implement this, we should see if adding this is on the ICU library roadmap. ICU is open source. It might be better for us to implement it there, and contribute back to ICU than implement it in Daffodil's code.
Similarly for the textNumberPattern P, V, and @ characters. (See DFDL-853)