I just bought Titanium Backup and set up a scheduled encrypted backup of user+system apps and data, followed by Dropbox sync. Assuming I remember the passphrase, is all the data required for restoration backed up? Or do I have to manually backup the private key and encrypted symmetric key?
You only need to remember the passphrase. I've done this before.
If you're worried, you could always create a nandroid backup of your entire system, then wipe and install a custom rom. Install TB, and try to restore an app with just a passphrase. If it needs something you lack, then you have your answer and can go back to recovery and restore the nandroid. But as I said, I have done this process already and know that the password is all that is needed.