It does require client modding, yes.
When we first researched the features file on the old forum, someone did find a feature that partially (de)activated the old GUI, 95b07f6e. I vaguely remember said someone saying that the client crashes on certain windows though, so I don't think that's a viable solution. The features file is fickle anyway...
The only alternative I see would be to mod the new GUI, to look like the old one as much as possible. The new UI is largely driven by XML files, in db/layout2, and the images that make up the windows and dialogues can be modified as well. It will probably never look exactly the same though, and it will be a lot of work.