Shrug All you really need is an optional contract clause to make it okay. Heck, one could make the argument that such an assessment is a method of preventing power harassment: By monitoring the mental health of an employee, the company is actually doing more for the employee him/herself. As long as such information isn't used against the employee (say, to fire or get the employee in trouble, or to justify not giving a promotion/not hiring the person), I don't think it'd be such a big deal.
Let's face it, corporations already discriminate based on assumed information about psych profiles. This would just be laying all the cards on the table. I can think of a crapton of suits being prevented on that basis.
EDIT: Unfortunately, such a measure would require all HR personnel to get some sort of psych certification, which would raise the average salary of HR personnel...