For me, it would depend on the flavor text (even though it is not part of the "rules", I do sometimes have the players read the flavor text to help decide if they can do something unexpected with a power). If it was like a Storm Warden's slide, which would be based on wind and thunderclaps, it's a force from the square they are in which is pushing them into the other square, so the "effect" would be emanating from the square that they started in. I don't know of any offhand, but there are probably some that have flavor text that may refer to being dragged by spirits or black holes or whatever, in which case you couldn't (but you could pull someone outside line of effect into a square that has line of effect, as long as there is line of sight to both).