I can't speak to B&H but Photocreative and Henry's in Mississauga and Aden Camera in Toronto let me try-before-buy. This is particularly true when I'm considering something a little more exotic. So far, my ZE 2.8/21, ZE 2/25 and ZE 1.4/35, ZE 2/100 and 400/4DO were tested in the shop.
I do it this way - I arrange to go at a quiet time, take my camera, use live view to focus, shoot some images and note the serial number of the lens. I then take the images home and look at them carefully. Then, if I like the lens, I'll buy it, if I don't, I'll show the sales people the images and they can use them to show the manufacturer why the lens was rejected. For the record, all of the lenses I've tested exceeded my expectation.
Of course, it helps if you have a good working relationship with the sales staff and you don't jerk them around. IMO, it would be completely unreasonable to ask to test (say) five lenses at the same time and look for the best.