Even assuming the Texas Eagle is on time, we don't depart St. Louis until about 8 pm. I was thinking that if I made an 8 or 8:30 pm reservation at around 6 pm while still riding in coach, I'd be more likely to get a dining reservation for that later time than if I waited for the SCA to obtain late dining reservations for the newly boarding STL sleeping car passengers. I'm thinking there's a good possibility there won't be time for these new sleeping car passengers to even eat in the dining car.
Could it be that the 8p-9p dining reservations are generally held so the SCA's can offer them first to the on-coming sleeping car passengers, so Chicago coach passengers can't even reserve during that hour? Just thinking through the possibilities.
While I note that you've already moved on to another idea, please let me explain how the above is supposed to work.
When the dining car LSA comes around to the sleepers to obtain dinner reservations for those already in occupying their sleepers, the sleeping car attendant is supposed to secure dinner reservations at that time for people boarding downline. The SCA is supposed to secure reservations that obviously match up with the times that the passengers would be boarding; in other words obtaining a 6 PM reservation for someone boarding in St. Louis as that would of course make no sense.
So what all that means is that you could have actually found out that the 8PM hour was sold out by the time the LSA got to you in coach simply because the sleeping car attendants combined with a few other people in coach had grabbed all the openings at that time.