I just nod and walk through the dining car when I want to access the lounge -- but I am usually not doing that during meals, as I will be eating in the diner during some or most of that time. You do know, don't you, that meals in the diner are included with your ticket for sleeper accommodations? Sleepers are considered "first class" accommodations. You'll receive everything at the meal including "extras" like breakfast meat, dessert, beverages, etc. The only thing that is not complimentary is alcoholic beverages.
Just bring your ticket with you, as you'll need to sign for the meal with your car and room number (and referring to the ticket is the easiest way to get that). They'll pretty much be able to tell that you are a sleeper passenger by the direction you enter the diner from, but you will *need* that ticket if it's your first time in the diner and you enter form the lounge car direction. After that, there's a good chance they'll remember you, ha, ha! They are good at knowing their passengers.