....I don't see it quite the same way. True, if you are in a sleeper you have not paid for a coach seat. However, many times I have been on a train in sleeper and stopped in a half-full coach car and sat for a while. No one has ever said one word to me about it.
I think the key, as you say Bill, is to be mindful that at station stops, people who have paid for seats are entitled to them.
I would not consider asking a coach attendant. Just be aware of how many seats there are, and how soon the next station stop is. That is my cue to get up and move to the cafe or back to my roomette.
And if an attendant notices you sitting with no destination tag above the seat, and asks you what you are doing sitting there, just smile and get up and walk away. As I said, I have never needed to do that.
On the other hand, on a crowded train, I would not try any of this.