OK, I have implemented it as a plugin. Download
change_video_width_plugin.7z. Extract the change_video_width_plugin.tcl file from the archive, and place it in the "plugins" directory (within the PgcEdit's installation folder).
You should have now a new Plugins -> Change Video Width menu, with an option to change the video width of the currently selected domain from 720 to 704 or from 704 to 720. The half width (352) and the PAL/NTSC heights (576 and 480) cannot be modified.
There is also a global function that toggles the width in ALL menu and title domains of the DVD. (The half-widths are not modified.)
You can also use the Change Video Width option at the end of the menu that pops up when you right-click on a PGC in the left pane. It is easier to access, and changes the video width of the current domain only.
I hope it's sufficient to fix the bugs of your authoring software. Let me know if you need to toggle the widths in all title domains only and not in the menu domains (or the opposite). I can easily change the global function to handle the menu and title domains separately.