UPDATE: I’ve managed to create a working version! See here.
I’ve written this excellent script that will replace specified background images with an SVG image, whenever support for this is detected. It will go through all CSS rules looking for “filename.svg.jpg” and replace it with “filename.svg”. This should be ideal for anyone who would prefer to use SVG backgrounds whenever possible.
Hopefully this will be fixed before Opera 9.5 goes final, if not, it’s back to the drawing board to write a script that’s more forgiving!