Hi,
Sorry for not getting to this post sooner. We're still recovering from Devcon
Zulu does respect FileMaker privileges, and this is actually handled pretty gracefully in iCal. If you attempt to break your privileges in iCal you get an error message that it's not permitted, and nothing is changed in FileMaker. If they choose to continue offline, then they'll be out of sync for a bit, but the next refresh will right things. The initial sync does have Zulu writing the Zulu_UUID back to FileMaker, so if the privileges don't allow this, then the events won't be recognized in iCal. This is usually not an issue as long as there's one account with write access that can populate the UUIDs. If all iCal accounts have this limitation, then the events in the past will generally be ignored by Zulu since it won't be able to write to them...but maybe that's a good thing in your case.
Google, unfortunately, is not as graceful, as it's syncing two data sources. We typically don't recommend using FileMaker privileges to restrict events when using Google. It will throw an error in the sync routine, and may prevent it from completing, but this is usually a one time problem and the next sync will go through. The problem is that the user was able to change the event in Google, and may not see it revert, and since the mod timestamp wasn't triggered in FM, the events can stay out of sync.
For restricting events in Google, we recommend pairing all calendars with an "Admin" account with write privileges, and then share them out to the different accounts as read only using Google's sharing functionality. This is also good because you only have one pairing between Zulu and Google, and have Google do the heavy lifting. Unfortunately, this is either on or off unlike a dynamic FM privilege.
The simple answer would be preventing dates in the past from reaching Google, but that may be too limited. The other option is to maybe use the Zulu_PostEdit script that runs when after an event is edited by Zulu. This is a web publishing script, so not all steps are available, but you could use this to revert records that should not have been edited.
Let me know if any of that is helpful,
-Jason