Can't comment on the 6pm-7pm period but I used the YYZ MLL showers at about 8am before the daytripper AC868 to LHR.

The showers are nice. It's a biggish bathroom (bigger than most hotel bathrooms). There;s no shower cubicle per se, more of a glass divider. Big mirror, sink, towels supplied. The showers are cleaned after each use and the towels replaced - there's only enough towels for one in there. The process is simple - you need to give in your BP at the reception desk, then the agent will let you into the shower room. They keep your (stamped) BP so that if they are looking for you (e.g. for an op-up!) and you are paged on the overhead tannoy, then the MLL agent will at least know where you are. They have also had problems in the past where missing pax have turned out to be in the shower.