I don't have a definitive answer for either of you, but on our recent trip, in general, I found Disney restaurants to be quite accommodating. BUT, with free dining, I think those same restaurants are going to be extremely busy and far less likely to be able to take special requests. Sometimes kitchens aren't set up to make certain items at certain times (think about McDonalds and breakfast only be available until, say, 10:30AM. Once the kitchen is changed over, that's it, no more breakfast.) I also think that dinner menu items tend to be more expensive; if the salmon is a more expensive item than anything else on the lunch menu, chances are they won't let you order it, even if the kitchen can make it. I think its far more likely to be able to get a cheaper lunch item at dinner than vice versa. We asked several times for a hamburger for my picky son at dinner time and didn't have a problem.
Sometimes the inability to order something makes no sense at all to patrons. But things go on in restaurant kitchens that we might not be aware of. We were at a very upscale steakhouse in downtown Chicago a few months back. I had checked menus online beforehand and remembered seeing hamburgers. We showed up at dinner and I didn't see them on the menu. I asked about it and was told they only serve them at lunch. I asked if it were possible to get one for DS and was shot down immediately. Now surely the grill was up and running because they were cooking steaks; but they said they couldn't do burgers and I didn't question it. I ordered plain spaghetti & meatballs off the kids menu for my son and he dealt with it. The restaurant and server both would have profited more if I had ordered a "grown-up" hamburger for him. Go figure.
Anyway, my gut feeling is that special requests will be much more difficult to get taken care of during free dining.