I did it in August after my Camino Portugués. My compañera wanted to do it - so we went purely on a lark! If we got in we got in, if not, oh well. It was comida - the middle of the day - and there were only 6 of us. You do need your compostela, but as we were only six, they accepted our credencial as that was all she and I brought with us. Had there been a full complement of 10 people we would have been turned away. The food was ok, a sort of mixed vegetable bowl to share, soup and a main course which I really can't remember - honestly, it wasn't memorable. The lark was having to get "checked in " at the parking garage door, then you go up to the main entrnace and they lead you through two beautiful courtyards to the kitchen. Up the stairs, grab a tray, get your meal and then back down the stairs to a tiny dining room with a rather well used table cloth. However, we got all the wine we wanted - that was fun - and we all had a good laugh. Clearly none of us "needed" the meal.
We had a unique experience in the evening though. We encountered three young Spanish guys with whom we had walked much of the Portugués. They were in their twenties and one was a special ed teacher - they were all on a pretty tight budget. They had met on the Camino in 2009 and decided to do the Camino Portugués together for the Holy Year. We told them about the dinner and took them to the Parador to show them where to go. As we turned the corner to enter the parking garage, an Australian (sorry, no offense to my aussie friends!) woman began shouting at us.... "No, NO, NO, we are already counted! We have been waiting here for 30 minutes and he has already counted us - There is NO room for you!" It was very shocking, really. Especially since Chantal and I had no intention of eating there again - we just wanted the boys (who could actually have really used it!) to benefit from the opportunity.
Anyway, I got some really nice photos of the interior courtyards and chapel of the Parador - but if I never got the chance to eat there again, it would be no great loss. It's fun if it works out, but dont' put too much stock in it.