Major Description
Elements in a hidden group have a restriction that they must define dfdl:outputValueCalc, a default value, or are optional. This is necessary so that they can be unparsed without appearing in the infoset.
It might also be useful to allow dfdl:inputValueCalc to appear in a hidden group. It seems a bit odd since we would calculate an element only to not include it in the infoset, but it could be useful as an interim calculation for another non-hidden inputValueCalc.
On alternative to a hidden IVC is use a variable, but in some cases infoset paths/elements are easier to use since you don't have to worry about new variable instances or varable scoping.