When OmniGroup first added Taskpaper support to OmniFocus they published an Editorial workflow which allows you to create new project - in the Mac Power Users Facebook Group someone asked if you could specify which folder the project should go into, a few minimal tweaks later and here is the solution! The key changes were:

• Ask the user which folder they want to save the project in.
• URL encode the folder name.
• Modify the X-Callback URL so the project would be saved to the specified folder.

You can get OmniGroup's original Editorial workflow from their blog post here, and the modification to allow you to add directly to a folder is available here: New OmniFocus Project in Folder