Yep, given that most industries are under provincial regulation, typically the Federal labour code isn't applicable except for specific industries (such as working for an airline). On the assumption OP's industry is provincially regulated, you'd have to look at that province's OHS agency. For example in BC it's WorkSafeBC.

In Worksafe's prevention manual the closest specific thing I could find that relates to travel/seats is transportation of workers in Part 17 of OHS Regulation. Meeting the regulations isn't that hard given that the requirement is 16 inch wide seating and, what I read as, 26 inch pitch.

That said, a more relevant section would be Part 2 which covers undue risk to a worker from hazardous activity not covered by a specific section of regulation. Undue risk means a greater than normal probability continued exposure to the work, or working conditions, will result in an injury or adverse health effect.

I don't think it'd be hard for someone that has a higher risk of DVT to get a doctor's opinion they're in undue risk by flying in cramped seating. But that'd just open a whole new can of worms. The employer has to find someone else to do the traveling; change job descriptions; and alter the interview question(s) around employee travel "are you willing and able to travel (in a flying metal tube going 500 mp/h packed in like sardines because we only pay for Y)."