I think that was a proposal for the MGM production of "Marie Antoinette". The screenplay was written in a way that made both Louis and Marie seem rather harmless and fun, and the writers may have considered having Marie say, "Let them eat day-old cake" to make her seem more callous.
Good try, but no cigar. I found the reference; it's in Edward Albee's THE AMERICAN DREAM, one of my favorite plays:
Mommy recalls that Grandma has always wrapped boxes nicely. When she was a child, left poor with the death of Grandpa, Grandma used to wrap her a lunchbox every day for school. The other children would withdraw their chicken legs and chocolate cakes from their poorly wrapped boxes, and Mommy would not have the heart to rip into hers.
Daddy guesses that it was because her box was empty. Mommy protests, saying that Grandma always filled it the night before with her own un-eaten dinner. After school, Mommy would bring back her lunch for Grandma to eat. "I love day-old cake" she used to say. Mommy ate all the other children's food at school because they thought her box was empty. They thought she suffered from the sin of pride. Since that made them superior to her, they were quite generous.
