After some further investigation into my own Heroku timezone problems, I found a post which indicates that you actually can specify the timezone at an application level, using the following command:
heroku config:add TZ=Europe/Oslo
I believe this may be the answer to all your troubles. Courtesy of http://www.reality.hk/articles/2010/01/07/1319/ (Edit: Broken link as of 2012.08.23. Archived copy.)