I don't know if that can to be you useful http://pinguyos.com/2015/09/pinguy-builder-an-app-to-backupremix-buntu/
A DVD bootable must contain a system for it. Also there are DVDs read only useful for conserve integrity of system or DVDs with possibility of copy more times which are more expensive, i don 't know if this class last DVD can help for it
And a question: how you get burn the image system trisquel together with data, modifying the image iso?
Look this tool too for customization images http://packages.trisquel.info/source/toutatis/uck and
On the other hand, maybe you are searching about "live cd or usb data persistent", i suggest search for this in the web
regards
