I'm going to tell you why this won't work. But, it's a great suggestion for people who are not competing, as a way for them to learn some of the difficult quotes.
I do crosswords on a site that shows incorrect letters as red. When I get stuck, I just stay on a square and type the alphabet until a black letter comes up. It's a built-in way of cheating, and there is a setting for those who don't want the help.
If folks here had a feature like that, it would be "worse" (depending on your point of view) than a hint button. There would be less true solving, merely eliminating wrong possibilities.
Slotted spoons don't hold much soup. - Stephen Sondheim