When your app is idled out, it is taken out of the dyno grid. The next time it boots, it will have a clean tmp/ dir.
As far as hooking the dyno wake, that would just be adding some code that runs when your app is booting. If you're using Rails, this could be a script in config/initializers -- You received this message because you are subscribed to the Google Groups "Heroku" group. To post to this group, send email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/heroku?hl=en.
