You mean for a Data folder on the disc?
I've never tried, but you should be able to save the disc as an image and then use this command on the command prompt:
hdiutil attach myDVDimage.img -readwrite
This will force the image "myDVDimage" to be mounted as writeable. Then you make your change and then unmount the disc.
Make sure you "cd" ino the right folder where the image is. before issuing the command.