Modifying these main structures will make everyones' session files broken (when you read an old session file with a new "win" struct) Can you use parameters? http://workgroups2.readthedocs.org/en/latest/guide/data-structures.html#parameters

Now it use win parameters. and the win struct is not modify.

Found another problem...

1. These parameters (that are saved) may contain complex objects (like #<buffer>). They cannot be saved "as is" like #<buffer> or you will get an error in reading a session file. You can see just a few lines below that I hacked it using (wg-pickel ...) before saving window-next-buffers.
2. I've met a new object for me for example. #overlay in no buffer. Didn't test anything, maybe it works with wg-pickel. For such sudden errors I used (ignore-errors...)

Have you met these problems?

PS: I know this extension is complex