This was from "wally" on 
http://m.blogs.sitepoint.com/2010/11/08/optimization-made-easy-with-mod_pagespee
d/

If this could be achieved using the Apache directives <Files> and <FilesMatch> 
in .htaccess that would be very helpful.  For example, to disable image 
optimisation for a CAPTCHA image or to turn off JavaScript rewriting for a 
specific directory.

I am starting to work on this now.

The initial support will allow selecting filters and thresholds on a 
per-directory basis, but this directory is only used matching the HTML file.

As a follow-on, we will work on allowing filtering for the names of resources 
to rewrite, e.g.

  ModPagespeedEnableResource images/*.png
  ModPagespeedEnableResource styles/*.css
  ModPagespeedDisableResource images/captcha/*

This was fixed with release 0.9.11.5.  .htaccess is support is detailed in the 
documentation:

http://code.google.com/speed/page-speed/docs/using_mod.html#htaccess