[Pharo-users] How to detect an inactive image?

At the moment I have several projects in parallel. Nearly all of them are web based. I use a cherokee server on my local machine that starts a squeak image whenever I access a project via web browser. For persistency I use piers image based persistency. But I don't want to have all of the images run all the time. I would like to shutdown images if they aren't accessed for some time. I could do it via seaside but then I don't want an image to shutdown while I'm working on the image and not accessing seaside. What is a good measure of activity inside the image? thanks, Norbert