Major Description
The DFDL Spec says dfdl:extraEscapedCharacters is a list of DFDL string literals.
Other properties for escape-character stuff allow expressions (dfdl:escapeCharacter, and dfdl:escapeEscapeCharacter), so the fact that dfdl:extraEscapedCharacters does not is inconsitent.
We will bring the issue to the OGF DFDL Workgroup for consideration as a spec correction/improvement.
However, as this is hard to work around, there's no reason this can't be added to Apache Daffodil as a DFDL extension, and documented as such.
Email thread of discussion from users at daffodil.apache.org list is here:
https://lists.apache.org/thread/yd1zo73sjf49v2wqnqwh0dwzy26k0v4c