I recommend looking further into the question of whether or not you can get the external optical drive to work in ChromeOS. As for Ubuntu, an external USB optical drive, like USB drives in general, should work without problems. To test this, create an Ubuntu live USB stick, boot a Chromebook from it, and see if the external optical drive works. (Don't install Ubuntu; instead select Try Ubuntu without installing and see if it can recognize the external optical drive.)
If you don't already have a Chromebook, you might or might not be able to persuade the staff of a store to let you test this. But since a Chromebook doesn't appear easily to do what you want it to do, this might not be the best computer for you.
Ultimately, USB drives of any kind usually work on Ubuntu without any problems, on any computer (so long as you can run Ubuntu on it). There is no 100% guarantee, however.