Beards and Respirators
Understandable as far as religious/ cultural reasons. Then goto step two. The cost of this is a lot higher than a negative pressure mask. As for how enforceable, if it is in an employment contract, and there is a good reason for it. (health). Hard to understand why it would not be enforceable.
I am no lawyer.
As to legal then the health and safety act requires an PCBU to take all steps to eliminate or reduce to the lowest practicable risks to health. It also requires an PCBU to provide safety equipment that fits. So if a person was not clean shaven and you gave them a mask to reduce a risk to health.....are you complying with the law? Another question for the lawyers.