Custom notes aren't included. Thanks for the reminder, I should look into getting notes in there as MOD objects. Unfortunately I haven't been able to add new Object Preferences, so I can't make a note object with customizable texts. Instead, I'll have to make a new object per text that's supposed to go on the letter.
The Custom Objectives probably refer to the new Scripting Action "SET HUD TEXT" that can customize the text shown in the topleft corner. You'll have to enter a number to reference the corresponding text though, see
http://text.farcry.info for a searchable list of all texts with their numbers. There are about 30000 lines to pick from, they're the same ones as I will use for making the notes.