12 hour layover from TYO, after clearing Canadian custom, is it possible to enter the domestic terminal with a YVR-YYZ BP and use the lounge to shower, then exit freely to go to city and come back after? I haven't flown domestic enough and can't remember if it's set up like all US airports where one can enter and exit the terminal multiple times during the day.
Yes, exiting the domestic terminal at YVR and reentering is no problem. The domestic terminal is basically the same as US airport. (Certainly US departures is very different; I don't know if this would be possible from the non-US international departures area.)
From Canadian Customs, you can enter the domestic departures area without clearing security, though it involved a bus ride the one time I did it.