If the LGA duty-free can put your booze is in a sealed duty free bag with the receipt inside, you should be OK:
http://www.catsa.gc.ca/duty-free-purchases
Haven't done it at YUL but it was fine at YVR. Or you can observe that the duty free prices at LGA are higher than they are at NYC liquor stores so just buy it ahead of time, stick it in your bag, and check it.