To be clear, US seats are not legal for importation, advertising, or sale in Canada unless they carry the National Safety Mark for Canada (which some US models may have). CBSA does not stop Americans and make them take out their car seats at the border because they are not Canada labelled, and they can use their seats in Canada. Use of a non-registered seat in a Canadian registered car could void Canadian insurance for the occupant, but if the driver is insured by a US carrier, that probably would not be the case.
Since Canadian Standards are higher than US, a Canadian seat should be acceptable but I would imagine that most Canadian seats would carry the correct US certifications. Check the labelling on the seat for any certifications from NHTSA, UL or ASTME (it's not clear from a scan of US sites who actually certifies seats in the US). Seats are generally not acceptable for use anywhere after 6 years from the manufacturing date.