An example of a mixed content element in MODS is accessCondition. Text or elements are permitted in accessCondition.
Generally our use case in the crosswalk setting is text, which should be straightforward to implement with a String value. The "conversion strategy" pattern will help here, as it the common default strategy would convert a standard string input into an appropriate "mixed value". However, users might pick another strategy at some point in the future.
Extra credit: It would nice if nested elements/text could still be supported in this kind of element, but that will be a tricky thing.