The comments are much appreciated. I assumed, too, that it was a bit unfeasible, particularly so close to Christmas with very few flights from BOS-YHZ, so I'll probably just suck it up and take the 6:45am YYZ-BOS flight and sit around BOS for four hours until the 12:15pm flight. No point in risking it. Maybe I'll try to weasel my way into the lounge for the wait...