The specification section OPC UA version 1.05 Part 6 5.2.6 describes the binary encoding of structured data types well. Unfortunately there is no hint for the encoding of structure fields of abstract types (e.g. UInteger).

There is a hint in the "General" Section of Part 6 5.1.7, where "Variants are used to story any value or parameter with a data type of BaseDataType or one of its subtypes', which basically indicates to use a Variant where no explicit encoding procedure is found. I guess Abstract data types belong to this category, but guessing is not specific.

I suggest to give a clear hint or description on how to handle fields with abstract data types in the corresponding structure-sections. It is also possible to define a dedicated section in this part, which also describes the hierarchical dependencies of these Variants (e.g. to UInteger fields only unsigned integer data can be written).
It is also necessary to check/update to the other encodings (XML, JSON), where this information is also missing.