I'm trying to convert my project to use XRC instead of hand writing C++ myself. However, I'm currently using a wxSashWindow in order to be able to split an unsplit wxSashWindow. Since wxFormBuilder doesn't support wxSashWindow, I have to add a panel to my wxSplitterWindow in wxFormBuilder, then manually add a sizer to the panel, put a wxSashWindow in the sizer, put a sizer in the sash window, then put my contents in that sizer. It works, but it's a lot of extra work. Whereas when I'm doing it myself, I can simply make the wxSashWindow be the contents of the wxSplitterWindow, then add my contents to the wxSashWindow. That's 2 steps instead of 5.
Alternatively, another way to at least improve on my current situation would be to allow custom controls to be the contents of splitter windows. I believe that wxFormBuilder will only allow "container" classes (i.e. wxPanel, wxScrolledWindow, etc) to be the contents of wxSplitterWindows. wxSplitterWindow will accept any wxWindow as its contents, so I'm not sure why wxFormBuilder doesn't do the same.