I don't know any  way to change the modes of a  file, or even of the
  current file, within emacs.

  As I  chmod quite often, and  especially the file I  work with, I've
  made up a small package that implements facilities for that.

  The main functions are :
  - file-modes-to-number: Convert "a=rwx,a+r,go-x,u+s,a+t" to 3574
    (06766), "777" to 511 (0777) and 511 to 511.
  - buffer-file-modes: Return the file modes of the file visited by a
    buffer or the default modes.
  - set-current-file-modes: Set the modes of the file currently
    visited.
  - chmod: Change the mode of any file.

  If you're  interested in this module,  feel free to  do whatever you
  want with it :-)

  Any comment appreciated !

  Code attached.