I thought UA favored YYR/YQX as diversion points?
Using YHZ would be more practical with preclearance there as well (though I have never entered the US from there, so I am not sure how a international to US connection would work there without entering Canada.)
Yes it's YYR/YQX...very few UA 757 diversions to YHZ.
At YHZ, for an International to US connection......you would have to enter Canada first.....then right up-stairs to US Pre-clearance. It's a small, convenient airport!
www.hiaa.ca
http://hiaa.ca/travel-checklist/travel-to-the-us/