I'm no diplomat, but I see no problem with approaching China and saying: "He's our citizen, he committed a serious offence that would attract serious penalties in both our countries, we fully respect your right to try and sentence him according to your laws - could you please consider clemency? Your laws provide for sentences less than capital punishment, as evidenced by the original sentence. If you have the power to commute his sentence, please do so!"
There is no argument in here as to WHY the Chinese should do that. Yeah, we can make the request, but it's beyond me what guiding moral principle you're expecting them to see in it. Yes, they have lesser punishments on the books, but if their courts have decided that death is the suitable one for Mr. Schellenger, why would they care what Canada thinks?
And, for the record, the reason I conflated Sean's choices is because, divided by the "or", neither of them really seems very effective at all, ie. say we don't like the death-penalty(why would the Chinese care about that?) OR say that we'd like clemency(without providing any reason why, apparently). But "Say we disagree with the death penalty and ask for clemency" at least joins the request with a reason.