That's what I like about this place - you ask and you receive (thanks to the Lost Boys).
After my post yesterday I found:
http://www.extremeplanner.com/
It still has more features than I would like, but it is the closest I have found yet to what I am looking for. Of course, it is not Open Source, but it is cheap at $149 + $40 / year maintenance. There is an online trial which is hosted by their partner company, so I think I will have a look...