<p><strong>Emacs</strong>:  <code>C-U (79) #</code> &raquo; a pretty 79 character length divider</p>

<p><strong>VIM</strong>:  <code>79-i-#</code> &raquo; see above</p>

<p><strong><a href="http://macromates.com/" rel="nofollow noreferrer">Textmate</a></strong>: ????</p>

<p>Or is it just assumed that we'll make a Ruby call or have a snippet somewhere?</p>

I would create a bundle command to do this.
You can take editor selection as input to your script, then replace it with the result of execution.  This command, for example, will take a selected number and print the character '#' that number of times.
```
```
python -c "print '#' * $TM_SELECTED_TEXT"
```
```
Of course this example doesn't allow you to specify the character, but it gives you an idea of what's possible.