To my understanding, Canada has yet to apply the exemption to Yemenis. As of you chances to get refugee status from airport that is risky choice.
At first you would have be to recognize by the UNHCR to apply as refugee to Canada, that's why this petition is in place so all Yemenis to be included in the exemption rules for G5.
someone could come to Canada as a visitor or holder of TRV/study permit/work permit etc. and claim refugee once his plane has landed at Canadian soil.