For reasons beyond my comprehension Windows.Storage.PathIO.writeTextAsync can only write to existing files, not create new ones on the fly.

writeTextAsync.coffee

  writeTextAsync = (path, text) ->
    # First try if an existing file can be written
    Windows.Storage.PathIO.writeTextAsync(path, text)
    .then null, (error) ->
      [_, folder, subFolders, fileName] = /^ms-appdata:\/\/\/(temp|local|roaming)\/(.+?)\/([\w]+\.?\w+)$/.exec(path)
      folder = "temporary" if folder is "temp"

      createSubFolders = subFolders.split("/").reduce((promise, folderName) ->
        promise = promise.then (folder) ->
          folder.createFolderAsync(folderName, Windows.Storage.CreationCollisionOption.openIfExists)
      , WinJS.Promise.as(Windows.Storage.ApplicationData.current["#{folder}Folder"]))
      createSubFolders.then (folder) ->
        folder.createFileAsync(fileName, Windows.Storage.CreationCollisionOption.openIfExists)
    .then (file) ->
      Windows.Storage.FileIO.writeTextAsync(file, text)

  writeTextAsync("ms-appdata:///temp/apifail/file.txt", "This is how Windows.Storage.PathIO.writeTextAsync should have worked in the first place")
  .done null, (error) ->
    logger.error("Could not write file")

writeTextAsync.js

writeTextAsync = function(path, text) {
      return Windows.Storage.PathIO.writeTextAsync(path, text).then(null, function(error) {
        var _, createSubFolders, fileName, folder, ref, subFolders;
        ref = /^ms-appdata:\/\/\/(temp|local|roaming)\/(.+?)\/([\w]+\.?\w+)$/.exec(path), _ = ref[0], folder = ref[1], subFolders = ref[2], fileName = ref[3];
        if (folder === "temp") {
          folder = "temporary";
        }
        createSubFolders = subFolders.split("/").reduce(function(promise, folderName) {
          return promise = promise.then(function(folder) {
            return folder.createFolderAsync(folderName, Windows.Storage.CreationCollisionOption.openIfExists);
          });
        }, WinJS.Promise.as(Windows.Storage.ApplicationData.current[folder + "Folder"]));
        return createSubFolders.then(function(folder) {
          return folder.createFileAsync(fileName, Windows.Storage.CreationCollisionOption.openIfExists);
        });
      }).then(function(file) {
        return Windows.Storage.FileIO.writeTextAsync(file, text);
      });
    };
    writeTextAsync("ms-appdata:///temp/apifail/file.txt", "This is how Windows.Storage.PathIO.writeTextAsync should have worked in the first place").done(null, function(error) {
      return logger.error("Could not write file");
    });