Is there an OSX app/way to batch rename files via a purely text interface?
Emacs can do this.  


Open Emacs
C-x C-f and open the directory. It will open in dired-mode.
M-x wdired-change-to-wdired-mode RET


You can now edit the filenames just as you asked.  Pressing C-c C-c will end wdired-mode and perform the modifications.  


  wdired-change-to-wdired-mode is an interactive autoloaded Lisp
  function in `wdired.el'.
  
  It is bound to   .
  
  (wdired-change-to-wdired-mode)
  
  Put a Dired buffer in Writable Dired (WDired) mode.
  
  In WDired mode, you can edit the names of the files in the buffer, the
  target of the links, and the permission bits of the files.  After
  typing C-c C-c, Emacs modifies the files and directories to reflect
  your edits.
  
  See `wdired-mode'.