If you get CalDAV working, you will support a ton of non-gmail/outlook providers for 'free' including those that are self-hosted. For e.g. runbox, mailbox.org, mailo, etc. all provide caldav to their users - some of these also support exchange but in my 'research', caldav was universal.

Yes please! CalDAV may be an overkill, iCal (which is a "read-only" protocol) would be fine for this device.

I've started working on it yesterday :)