Here is a workflow for the iOS Workflow app. It is a share extension for adding emails to your TaskPaper file. I call this from the share action in the iOS mail app called Airmail. The workflow will create a new task at the top of your TaskPaper file with the email title/header as the task name and a URL as a task note for opening the email in Mail.app on Mac or iOS.
You’ll have to specify the Dropbox path to your TaskPaper file in the last action of the workflow.
You need the Workflow iOS app and the Airmail iOS app.