I haven't made an Automator script in a couple of years but it should be very easy. Basically drag and drop the actions you want. Then save as an application. Add application as a login item. (I'm not really sure if this works in Mountain Lion etc).
Yes, it's very easy. Open Automator. New workflow. Choose music Actions. Add "get specified iTunes item" and choose the playlist you want Add "play iTunes item" Test the workflow "play" button Save... Select "application" from popup
Done! Mine took two minutes via remote access using iPad in bed. Very easy :D
Ok, I have had a chance to get this installed, but the problem now is that while it loads te playlist, and I have included the command to play the items it does not want to start the music.. :-(. But at the moment it's just a matter of clicking the play button, I just wanted to get rid of the human requirements.