@xx not really but something that comes close to it.
Use the spoiler plugin made by @Nicolas. Spoilers are only viewable by logged in users.
I'm currently working on a calendar / events plugin, but so far, it looks like I would have to hack it in via a hidden category and a widget that would show the calendar, or do it all manually.
I haven't seen any hooks related to this. Are there any?
It would probably be easier for you to build your own system rather than use the posts API to build your calendar plugin (I think so!). If you decide to go this way, can you elaborate on how you are doing this then maybe we can help by adding hooks where you need it etc