I tried several methods using various UI and CLI tools for Linux and Windows, but to no avail. Maybe there's something I'm missing, but maybe it could be a limitation on Dual-Layer disks. I did however manage to get my 13.37 copy upgraded to current, but it took some time.
I had to run slackpkg with the switches for update first, then clean-system (to remove outdated and removed packages), run install-new (to add new packages for dependency resolutions), and then run upgrade-all to finally get everything up-to-date.
I'm going to see about getting a 8 GB flash drive soon so I can make a USB installation drive.
Thanks for the help, I'll mark this as solved but if anyone has any methods that have allowed you to burn DL media feel free to add them.