I have received a prompt written reply to an inquiry to Transport Canada (thank you):
Division 4 (Sewage) of the Regulations for the Prevention of Pollution from Ships and for Dangerous Chemicals (under the Canada Shipping Act) stipulate requirements for pleasure craft in Canadian waters.
With regard to specific requirements for toilets, they fall under provincial legislation and are unrelated to Transport Canada [emphasis added]. You will need to contact the appropriate provincial ministry for further information.
So, it sounds like one province might establish regulations that prohibit composting heads, while another permits them. I wonder what the story is in the Maritime Provinces.