For SMS there is an application called SMS popup which will display the message over the lock-screen.
Is there anything similar for calendar notifications that allows you to clear or snooze the notification without having to unlock the screen?
WidgetLocker is a great lock-screen replacement that will allow you to put any widget installed on your phone on the lock-screen. You can also configure it to allow your Notification bar to be usable. The actual calendar popup I'm not sure about, but I know you'd be able to see it when it shows up in your notification bar, and from there you can open it up and snooze/dismiss it.
This works: Your Calendar Widget