[SVG 1.1] BackgroundImage and alpha compositing

Hello,

I've run up against a problem with filters and the use of
BackgroundImage in SVG 1.1: the filter output is alpha-composited with
the background, which results in double-compositing if the filter itself
composites with the background (e.g. when using feBlend to get "blend
modes").  This is noticable in areas of the background which are
semi-transparent, which may be due to transparent objects, or simply
antialiasing.

See here for an illustration of the problem:

 http://www.rydia.net/~mental/www-svg/background-image-alpha-issue.svg
 http://www.rydia.net/~mental/www-svg/background-image-alpha-issue.png

Is there a way to work around this?  Ideally there would be some way for
filter output to replace part of the accumulated image rather than being
composited with it.

-mental