You can not do what you ask because, in general, you can not set a custom search engine as the default and thus have it appear on spped dial.
However, you can add youtube's search as a custom search engine. Right click on youtube's search field > create search engine > fill the details there > done.
Perfect! I used the 14.04 package located at http://packages.ubuntu.com/trusty/chromium-codecs-ffmpeg-extra
instead of the 12.04 I'm running. It did break the chromium browser package, but I don't use that anyway.