Unfortunately, message boxes are generated by a call to a system routine, not an Access-exclusive routine. This is a Windows setting, not an Access setting. But the thing is, these folks should ALSO have trouble reading icons and file names and such. Same type face and font size are used for both.
Look at the Windows "Accessibility" settings (if you have such a thing) on your workstation Control Panel. Even on a system with tight security, the Accessibility settings should be accessible. If not, you might even have a disability harassment complaint on your hands.