Of course it is a reality, and a simple one too. It would all work through the update manager. These Ubuntu-Android builds would be treated how the various live Ubuntu version (11.04, 11.10, 12.04, 12.10), each having their own specific updates.
I assure you though that Ubuntu always strives for simplicity and ease of use, especially when it comes to installation and upgrades.
You can follow all the progress for Nexus 7 Ubuntu at HERE and HERE, all of your questions will be answered.
Please keep in mind the this type of question fits firmly into this statement listed on the FAQ:
"You should only ask practical, answerable questions based on actual problems that you face. Chatty, open-ended questions diminish the usefulness of our site and push other questions off the front page."