I'm assuming by "fault" you mean a servo fault and you are using a Gecko 320 to drive it. The doc for that drive (
http://www.geckodrive.com/photos/G320_REV-4_MANUAL.pdf) discusses how to access its ERROR output and still allow the drive to be reset via a switch. If you've got an output, and enough inputs on your breakout card, you can presumably send the result back to Mach and set up Mach to shut things down on a fault, much as it would if you hit a limit switch. This will require a bit of macro work as well as a bit of electronics, so you'd best get some help on the Mach and Gecko support boards if you really want to pursue the option.
Best,
BW