I notice in the OP that Reba approached the hostess "Shortly after dinner had been announced" which makes the timing unclear. Was dinner finished? If Reba was asking before the hostess had eaten her own dinner then I think it woud definitely be rude. I think the timing is very important.
I think by saying "Dinner was announced" I made it seem more formal than it actually was. People had gathered in groups in different areas of the house - the dining room, kitchen, and a couple spots in the finished basement. The host and hostess, once dinner was ready and put out, went around to each group and said that dinner was ready, at which point everyone began to move toward the dining room. However, it wasn't a mad rush - the group I was in, for instance, saw the other group close to us all get up and go upstairs (we were in the basement), and so we waited a bit to give everyone else time to go through and not all crowd into the dining room and make the people ahead of us feel rushed.
I believe Reba and Bill were already upstairs, so were among the first to have dinner, and were finished with it even though some others were still making up plates. If I remember correctly, the desserts and various sweet snacks were sitting on the kitchen table. I'm not sure if the particular dessert Bill wanted was sitting out with them or not; I know it had been brought to the house in a closed container (it was a pie, and it was in one of those plastic rubbermaid/tupperware pie containers, which wasn't transparent. So if it was still in the container and not on the table, I don't know how Bill knew it was even there; but then again, if it was sitting out on the table, would that have been reason enough to think it was ok to ask for it? In other words, would it be ok to assume it was there and being offered?)