Now that the release is done I'm going to take another look at this. I believe that if an NSCustomView is being encoded it should probably be converted into an NSView when it is decoded by the nib loader.
I believe that this change will solve your problem. I'm going to make a change to GUI and send you a patch to see if that helps.
As an aside, I recall you suggested I test in Gorm. I have come to a conclusion on this as well...
Flat nib files like this one can't currently be loaded in Gorm because the loader doesn't have access to the metadata it needs to determine custom classes and other information which is normally in classes.nib and info.nib in older, pre XIB nib files. The classes.nib and info.nib files both contain certain mappings which are needed by Gorm to do the editing. Without them Gorm has no way of knowing what the superclass of the custom class is. The keyedobjects.nib file contains a pointer to the className of a custom class, but it will not provide the super class of that class. This is what the classes.nib file is for.
Because of this I don't think that Gorm will be able to edit newer nib files beyond those created with 10.4/10.5, which is unfortunate. So the course of action in Gorm should be to perfect XIB handling in Gorm and also perfect the nib handling for the above mentioned versions of Mac OS X.
All of that said, gui, since it doesn't require the same metadata that Gorm does since the above mentioned files are only for editing, should be able to load these files just fine once I've made my changes.
I will report back here and post a patch for you to try.
Thanks, GC