I'm probably going to get some flak for posting this, but I have a 100% success rate for sneaking into showers in the sleepers as a coach passenger. The dining car staff, who act as guardians of the sleeper section, don't remember every single sleeper passenger, of course, especially those that just got on. The trick is to confidently and purposefully walk through the dining car, don't make eye contact, and walk directly into the sleeper car. Once there, you may run into the sleeper car attendant, if you're really unlucky, but again, chances are he won't stop you. Worse case scenario, you may have to tip him $10 for the privilege of using the shower.
I don't feel too bad about doing this, because the showers in the sleepers tend to go unused anyway.