Thank you @Derek_Rokicki for the info. I’m on an older IS version and hence have to decompile the definitions myself.
But I’ve already solved the problem. The statement “they cannot be read directly via the protocol buffer APIs” is half true. The truth is that they can be read (otherwise the IS would not be able to read them as well), just with some tricks. The keyword here is “unspecified fields”.
If you’re familiar with protobufs etc, it would be cool if you could answer my question from another thread. Thank you!