What i do is tar my changes folder after every major update, and test install everything new in the default build.
However i understand where your coming from and to me it looks as tho you can just copy everything in the changes folder over to the matching directory of the default bt4 dvd with an iso tool. That should work just keep in mind to overwrite everything as whats in the changes folder is going to be the data you want to keep.
Like i said i haven't tried that but logic says that it should as its my understand that everytime you modify a file from the default build its saved in the changes folder with a matching directory.
There is also a guide JUST posted on on how to load the live dvd and apply changes and then rebuild it. Its on the Offsec blog if you want to check it out but im not sure if it only applys to the pre-final.