The one instance I can think of where coach passengers were barred was the Super Chief/El Capitan which Santa Fe treated as two separate trains that happened to be coupled together. El Cap passengers were strictly barred from the Super Chief section, but the El Cap had its own full diner as well as the Kachina Coffee Shop on the lower level of the lounge car.
That was one example. The two formerly separate trains were combined due to a downturn in business from around 1968 until Amtrak day, except for peak season and holiday periods where they were separated into sections.
Another similar example was the Panama Limited and "Magnolia Star" during the same period.
A different example was the seasonal ACL and then SCL Florida Special. After coaches were added to it, they ran separate diner's for Pullman and coach passengers. The feature 'Recreation Car' was open to all.
Most of the feature western transcontinental trains had a full diner, and a separate lunch counter type diner, but both were open to all passengers.
PRR had some all-coach streamliner's that had twin-unit diner's...the Trailblazer and the Jeffersonian, on the New York to Chicago run or New York/Washington to St. Louis run, respectively...
There were many other examples of diner hospitality toward coach passenger's....