I’d take a look at Editorial for an iOS app that supports TaskPaper tasks as well as text editor. I have also used Taskmator for iOS. I’m looking at a new workflow now that will use TaskPaper again.
The biggest downside I’m seeing here is that there is no native first-party iOS client. Because the taskpaper file format is simply a set of rules around how to mark up a plaintext file, though, I could use something like Drafts or iA Writer for my mobile needs