Can an employer discriminate against me because I'm "ugly"?

When I teach employment discrimination law, my undergrads are often surprised to learn about all of the grounds on which an employer is permitted to discriminate in hiring or in other conditions of employment.  It is only the designated grounds in human rights legislation (or analogous grounds read into the statute by tribunals or courts) that are protected.  So we spend a lot of time in class discussing scenarios.  
Can an employer refuse to hire me because I dye my hair purple, or because it doesn’t like how I dress?
Can an employer give a promotion only to employees who are Toronto Maple Leaf fans, or who happen to wear blue on the day the promotions are decided?
One student this term asked whether an employer can choose women employees on the basis of ‘cup’ size?
The answer is ‘yes’ to all of these questions, unless the employee can somehow fit themselves into a designated ground in human rights legislation (or unless the employment contract prevents the employer from doing these things, which would be rare in a non-union workplace).  From an HR perspective, these may be stupid ways to make decisions, but they do not amount to a violation of human rights laws.  Some of the employees in the scenarios above may be able to fit themselves into a designated ground:  dress might be related to religion;  hair color might be determined by race (although I cant think of race that has a high degree of purple hair, etc.)
One scenario we deal with is attractiveness (or ugliness).  Can an employer hire only employees it finds to be attractive?  Can a bar hire only ‘beautiful’ servers?  Students are often surprised to learn that the answer is ‘yes’, unless the way in which the employer defines beauty excludes people on the basis of a designated ground.
As this entry from the American Workplace Prof Blog describes, researchers have begun to look into discrimination on the basis of attractiveness.   Apparently, ‘attractive’ employees earn more than less attractive employees, and are considered smarter and more honest.  A number of studies have also shown that professors considered ‘attractive’ by students receive better student evaluations.  Yikes! I’d better start spending more time on my hair!
Do you think the law should protect against discrimination on the basis of physical appearance?  How do you think that would work in practice?