Yeah, you are correct, Rocky.  It's got nothing to do with customs but you have to get a permit from the ranger station.  There's no customs at PP, so the RABC is your ticket into Canada but you still need to get your ticket into the park.

An exit via Horse can be done though.  You have to jump thru the proper hoops on your way in but you can exit any old way you like.

Theoretically, it might be possible to take a motorboat up to PP, pick up your permit, then motor back out and drive around to Mudro or someplace like that to start your trip.  But you'd better make it to the Q on the first day.  I don't really know, though.  Just conjecture on my part.