BG uses a fully external level format. This includes enemy sprite sheets, and object and enemy behaviours. It wouldn't have been possible to create the game without that. Now, the editor is not user friendly, or intended for anyone else than myself, and it's very difficult to configure object behaviours as each object is defined with a whole bunch of parameters. Many devices, such as puzzles or sound generators are programmed as Multimedia Fusion 2 sub-applications (which I'll provide the source to). You won't be able to modify those without Multimedia Fusion 2. The main application's source will not be available.
What I'm saying is that level editing will be completely unsupported and not recommended. It will also be very, very difficult, most of the time will be spent using the text editor. I don't expect to see any levels at all with their own puzzle devices and sounds.
Now, don't get the wrong impression. I'd love to be proven wrong!