All of the places I have ridden around Whistler are either free (no sno park) or you park at a sno park , such as Brandywine and pay the per day use fee (per sled).
I have never been asked for registration for the sleds and I have had the trailer searched before. Would be a good idea probably to have a letter with a phone # just in case they really did want to look into the sleds at the border and found out they were not yours.
Have fun, Whistler kicks A$$. Best place to ride IMO. Tons and tons of great riding there from big bowls to fun tree riding to great natural features to jump off. Hopefully the conditions are good. It is an epic place on a blue bird day with some POW.
You may want to plug this # in your phone just in case. As you can see from my avatar pic, sometimes bad things happen after a 2 minute long side hill...
Blackcomb Helicopters 604-938-1700.