I love that idea too!! DS has changed so much this year, he's a very "late bloomer" and is finally really starting to... well, become something. *I* can look back and see how far he's come, but it's harder for him to really comprehend it. A 'time capsule' just sounds fantastic.
We also school more or less year-round so have never done a real 'first day of school' thing. This year will be the closest to a 'first day' ever, since a few weeks ago we just happened to finish 3 or 4 curricula (or a level within a series) at the same time, and now he's in the final week of 3 straight weeks of summer camps... so when we get back into it, a lot of his stuff will be brand new or the "next level".
Before we moved a couple years ago, the homeschool group we were part of had a "not back to school picnic" every September. It was great fun! I think if we were to institute some sort of ritual, it would not be to mark when WE start school. Because we don't.

It would be to mark the day that everyone ELSE starts going back to public school, but we aren't.