I just created and uploaded a new Launchbar action as suggested above. You can get it from my GitHub account: https://github.com/raguay/MyLaunchBarActions
This action is used to add a new task to a TaskPaper project. If you send a path of a TaskPaper file to this action, it stores that path to add tasks to. Otherwise, if TaskPaper is running while you run this action, it will get that files path, store it, and use that file. If you run the action again when TaskPaper isn’t running, it will use the last stored file name.
Once it has determined what file to use, it asks the user for the task line and which project to add it to. The user simply types the name of the task, and selects the project. That new task will be added to the top of that project.
This action just modifies the text file. It doesn’t go through the TaskPaper API. I did it that way so that TaskPaper doesn’t have to be running.
Let me know if this works well for your use case.