For the others, this is about this answer. @OP: I assume you've seen the top-voted answer there. It points to a specific legal document confirming this request is legal. As my comments mentioned you can just link that. Any reason that wouldn't work?
How would I provide sources for a statement like "wearing pants to work is perfectly legal"?
You would simply say "wearing pants to work is not illegal". You could back that up by pointing to your familiarity with working in the jurisdiction the question is about. You could also back it up by pointing to relevant documents proving that not wearing pants actually would be illegal. Either experience or references would work for most situations. I believe in a courtroom that's often referred to as "evidence or authority".
The problem is that when you specifically state "I am not familiar with the laws in [country]", you cannot then go on to say "X is legal there" just because you think it should be. By your own admission you have no way of knowing. This is why the back it up rule exists. There have been a number of answers on this site downvoted because they pointed out that the law actually allows or prohibits certain behaviour that people felt shouldn't be.
If you are unable to corroborate a legal claim, it's typically a sign you shouldn't be making it.