using svg effects from user agent stylesheet.

Hi,
I've just discovered svg effects for html content[1], and I'm wondering if I
could use filter and <svg:feColorMatrix type="saturate" values="0"/> is an
user agent stylesheet. My goal is to have images in grayscale for people with
vision problems.

But if try to use something like 

img {
    filter: url("bnw_filter.svg#filter");
}

in userContent.css, or another css file loaded with nsIStyleSheetService, I
get following error:

> Security Error: Content at http://www.mozilla.org/projects/minefield/ may not
> load or link to
> file:///home/arno/.mozilla/firefox/vs5fvd6p.test/chrome/bnw_filter.svg.

I've tried to think of a way to use data uri [2], but I did not succeed
(probably because filter url needs to be inside an outer <svg/> tag).

Do you have any idea about on how to accomplish what I want ?

thanks
arno

[1]: https://developer.mozilla.org/web-tech/2008/09/15/svg-effects-for-html-content/
[2]: http://en.wikipedia.org/wiki/Data_URI_scheme