Question: When I start emacs, I can use the --title= option to control the title of the x-window that holds the emacs application. Is it possible to change the title after emacs starts from elisp?

Answer: 
I use

```
(setq frame-title-format "%b - emacs")

```

to include the current buffer name in the frame title.
The following worked for me (GNU EMACS 24.3.1 on cygwin multiwindow X11):

```
 (set-frame-parameter frame 'title arg)

```

which I wrapped in an interactive function

```
(defun set-frame-title/ag (title &optional frame)
  "set frame TITLE of &optional FRAME defaults to (selected-frame)
aka C11 window titlebar name"
  (interactive "sframe title: ")
  (set-frame-parameter frame 'title title)
  )

```