You may be able to do it, but is it worth doing it? What do you get over and above what Raspbian already gives you? Given that something like Unity won't work with any acceptable speed ( I think), and that Canonical no longer support any codebase with Armv6, so you would have to do all that yourself (ie. all the packages)
The reason Canonical dropped support is that it requires a lot of effort to maintain all the infrastructure need - all the servers etc, plus all the testing and maintenance.
Personally, I think Raspbian does everything required on the Raspi. Ubuntu wouldn't not give any benefits over and above what you already have.
All that said, if you want to do it, go for it, it will be an interesting learning experience and well worthwhile documenting the process.
Principal Software Engineer at Raspberry Pi (Trading) Ltd.
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