So, at long last, we've made a decision to license Zimbra for our 3000-some users, just as UW-Milwakee did for their 65,000 users. See
https://apps.carleton.edu/campus/its...dar_Report.pdf and
https://www4.uwm.edu/imt/about/news/zimbra/index.cfm
Carleton College will be engaging Zimbra's professional services team to, essentially, determine the "right" answers to the various questions posed in this thread. To the maximum extent possible, all work done will be released under licenses at least as liberal as Zimbra itself. Everyone benefits when common solutions can be adopted.
Our likely strategy will include designating our Moodle installation as the authoritative source and/or aggregation point for all course-related calendar and task data, so that professors need only make changes using appropriate Moodle tools, and the changes will cascade to each student's Zimbra calendar using iCalendar or XML protocols chosen with Zimbra professional services' help. There's some likelihood that some small amount of new code will need to be written on the Zimbra side to handle course adds, drops, and changes. That code probably won't be appropriate for general distribution along with Zimbra, but we won't consider it proprietary.