In all honesty, although this isn't what you're looking for, if the writers wanted to bring Amy and Rory back, they would. There are openings for loopholes.
In the episode "Water on Mars" the Doctor changed a fixed point in time. He had read articles on the subject of the explosion on Mars, and it was supposedly "fixed" but he changed aspects of the point. This shows it is possible, therefore that wouldn't have to be what would stop him from saving them.
Also, there are loopholes in the grave. As I've seen mentioned, all the graves meant was that someone named Amy and someone named Rory had to have a gravestone at that cemetery. They could buy a plot and plant the gravestone if they really really had to, but again, time can be rewritten, and according to "Water on Mars", so can fixed points.
Next, River's chapter title "Amy's Last Farewell" is vague. Amy and Rory say goodbye for the last time. That's fine. But The Doctor could have at least saved them and put them back in their ordinary lives in England, and he could've visited if he wanted. (I feel it's redundant to keep mentioning my could've changed a fixed point theory so just insert it here).
Finally, and most obviously, couldn't the doctor have landed In 1940 NYC or something similar instead, and picked them up? Or had them walk outside the city? Changing the fixed point in "Water on Mars" didn't cause a paradox, time just rewrote. Why would it cause a paradox this time?
My point is, there's several loopholes (though I might not have explained them correctly, I'm tired and it's late, though I wanted to see if there was a more definitive answer out there for this question that I missed) where if the writers (and respective actress/actor) wanted to, they could bring Rory and Amy back. The Doctor has pulled many crazy stunts throughout the series, therefore almost anything is possible if it's what the story calls for.