Not sure if this will help, but we use a small web-part add-in called SharePoint Reminder. It works on SPS2003 and MOSS2007. Since SharePoint calendars only alert on new or modified event, it didn't work so well for our users. SharePoint Reminder actually send an email to one or more recipients when an item is coming due.
The implementation is simple and the configuration is pretty robust for a small add-in. I believe you can get 10 reminders for free and anything above will cost you. I have a simple calendar that alerts an admin on upcoming contracts. Like I said, I'm not sure this is close to what you're looking for, but it can't hurt to check it out.