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:

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