Yes, the solution I'm using is working. Essentially the steps are the same, except that I only have to worry about NVIDIA GPU's, so I'm using one of their proprietary tools instead of lspci. I use an rc.local script to copy pci bus info to the xorg.conf file before lightdm starts.
I believe the issue you will have using lspci in a similar configuration is that lspci returns pci bus information hexidecimal format and xorg.conf wants to read a decimal format. So you can use lspci for your matrox card, but you'll need to change the base of your bus information before passing it to xorg.
One practical note: If you dont have to support multiple hardware configurations with a single image, don't do exactly what I did. Instead, I recommend simply using lspci to grab bus information, convert it to decimal yourself and simply write the xorg.conf file once.