No go for me, couldn't set the custom resolution...it sends all of my screens out of range. Could be ultramon or my 4th screen, but I don't have time to mess around right now.
I'm staying on 180.43 for the moment, 3x1280x800 is an annoyance, but frankly having working bezel management back makes it worthwhile.
Whenever you get the chance and have the older drivers with 5040x1050 res working (Custom input by GXM in the NV control panel), edit (goto Advanced) the setting and confirm its the same as above. Its possible the setting is automatically tweaked per system (specific monitor settings) and my exact settings may not work on all screens (though it should). This is unlikely IMO.
Afaik, the Matrox Custom res is just what you see there however, so if it works in driver version x, it should work in driver y as long as the settings are an exact match.