GetFilesAndDirectories
GetFiles
GetDirectories
I had offered these functions to Epic using their own system but they did not accept it
I use my own version of these functions all the time, seems pretty essential to me 
You can see the code I offered to Epic here:
GitHub Link
https://github.com/EpicGames/UnrealEngine/pull/23/files
You could add this to your version of the Engine to get this functionality in a Multi-platform compatible way
Or, you can do what I do, and copy out the FunctorFileDirectoryVisitor as a separate header file that you include in your own code, and then copy the functions into a static function library for use anywhere in your CPP code base.
I am still very impressed by all that you achieved!
PS: if you or others have sufficient interest in this I could try resubmitting the pull request with new and improved code and see what they say
I entirely rely on my own version of GetFiles and GetDirectories