What can you export the calendar as? Do you have a server you can run a regular export script on, make it public via http or something and then get your googs to pick that up?
Aha, fair enough.
Is the platform completely proprietary or is there a chance that someone else has worked on it? e.g. if it's Exchange/Outlook there's more of a chance of people knowing things.
If you're willing to use an unsecured intermediate why bother being secure at all?