It would depend on how busy the kitchen is and if they still have everything prepped for it. I know in the past we've asked about lunch entrees for dinner at Le Cellier and Coral Reef for one of our picky eaters and were refused both times. The reason we were given each time is that they prep lunch menu items early that morning and once lunch is over they put them away and prep items for the dinner menu. I think another reason though is that normally dinner entrees are more expensive than lunch entrees and they'd rather you order off the more expensive dinner menu. It wouldn't hurt to ask, just expect the answer to be no and be pleasantly surprised if it's not.
Just FYI: we've also asked for a dinner item for lunch once at Mama Melrose and was refused. They said the ingredients for it were not prepped yet.