Sorry for the delayed response. We did some thinking about this and it seems like a good idea. We could add the option to set an collapse element’s status in a cookie. We’ll play around with this idea a little and get back to you when we have something to test. Good idea, and thank you for the feedback (and your very nice review you left for our humble plugin!)
We are beta testing our Collapse-Pro-Matic Premium plugin that includes exactly what you are looking for. Please contact us at info (at) twinpictures (dot) de and we will happily include in our beta test.