Allow serializer/deserializer interface to use intermediate format instead of files

I want to serialize a property group to an intermediate ini object and manipulate it before writing it to a file.
I want to deserialize a file to an ini object and manipulate it before parsing it as a property group.

Use case:
I have several different types of proerty trees and want to encode in the ini file what type of tree it is.
Currently this is only possible with a lot of warnings and a 2-pass parsing.