Dashboard Widget for persistently editing a file
Dashboard widgets are simple HTML applets, built using HTML, CSS &amp; Javascript. So if there is a way to create an HTML webpage that does this functionality (namely, auto-loading a text file in an editable field with an option to refresh and save), then this can also be accomplished within a Dashboard widget.

It may be worth asking some of the folks at StackOverflow if you need help learning the code to accomplish this (web coding is better suited to that community), and you can use tools like Dashcode or Hype (my recommendation) to actually create your widget.