Basically, I am having huge difficulty getting a text file to open by my UWP app. I have set the app permissions to be able to access files in \'my documents\' and other opt
5 months later...
It appears Microsoft have now added the capability. Simply add the 'broadFileSystemAccess' capability to the app manifest as described here: https://docs.microsoft.com/en-us/windows/uwp/files/file-access-permissions
Note that this still requires user input to some extent (the user must grant permission to file system access on first run of the app) but that the file/folder picker UI is not needed.
I haven't actually tried this yet but it sounds like it should do the job. I hope it helps people referring to this post in the future.