I suspect that you got that error because you had a BWSplitView in a nib/xib file that you were attempting to load. In order to unarchive the objects in a nib file, the runtime needs to be able to create instances of the archived classes (e.g. BWSplitView). The exception that's being thrown is because BWSplitView isn't available to the runtime. In an Objective-C app you would link to the BWToolkit framework and the dynamic linker would do the work of making BWSplitView available to the runtime. In a PyObjC app, you have to explicitly import classes that you want available to the runtime (that aren't linked behind the scenes for you, such as the Cocoa classes). Fortunately, BWToolkit has a bridge support file so you can import it directly (assuming it's in a standard framework location such as /Library/Frameworks). If you need to load a framework that does not have a bridge support file, you can use objc.loadBundle and then use NSClassFromString to get a class object.
On a side note, /System/Library/Frameworks is reserved for Apple-supplied system frameworks. You should not put third-party frameworks in that folder as Apple may wipe that folder at a system update (unlikely but possible). Thrid-party frameworks that are made available to all users on a system should be put in /Library/Frameworks, user-specific frameworks similarly in ~/Library/Frameworks and application specific frameworks in Contents/Frameworks, where within the application's app bundle.