The way I do it (without having to modify a 1.14 patch and importing the .cfg files) is like this:
1.) Find a patch_mp.ff mod menu that has the CFG infection in it.
2.) Find a buttons_default.cfg menu, and put the .cfg files on the root of a USB drive.
3.) Insert the USB drive into the furthest right port on your PS3.
4.) Load the patch, load MW2, select the CFG infection from the menu, then back out.
5.) You'll see the 'bind BACK exec dev_usb000/buttons_default.cfg' or something along those lines.
6.) Go to a split screen match.
7.) Press back and then back out (this will execute the cfg)
8.) Go to a private match, and your CFG menu will have loaded! =D
Warning: this works, but if the patch_mp.ff uses the same bind to open the .cfg menu, you may have some conflictions, so either edit the cfg to use a different bind to open the menu, or just make sure the cfg you've downloaded uses a different bind!