In order to do some house-keeping here is what went on with the issue.
In fact I despised the approach of changing existing classes upon loading. From the perspective of .NET a DynamicMethod with the owner set to the type being extended would do the trick. However, you’re not able to add data object this way – if that’s a requirement, you need to derive your own class by Reflection.TypeBuilder. Both concepts are not available in .Net Standard (the former PCL).
I strongly suggest you get in touch with the DLR (Dynamic Meta Objects) in the case you have similar requirements – that’s the concept I finally started using.