I was looking through and I saw some formats for Twine 1 and the sugar-thing, but nothing came up for Twine 2 Harlowe. Sooo... since my brain was starting to cry trying to figure out where I was and wasn't closing things, and trying to read things in black-and-white.. Here you go.
It isn't great, this is my first time trying to make a user-defined-language for Notepad++ (and obviously I'm a newbie so I'm sure I'm missing a lot of Harlowe's capabilities, but its a start. If I tweak it over time I'll upload more versions, and if you have something better feel free to share it here.
[EDIT] I added a bit more functionality to it / supports a bit more syntax highlighting. Not everything, and focused on the scripting side instead of the string formatting side.. if you want to play with your own version I've also included a sample file with most of the markups from Harlowe's site, so you can see how it currently behaves. Enjoy