Keith Blackburn did a paper last semester on Romans 12:20 on that well known expression about "heaping coals of fire" on the head of your enemy. He had heard a nice "this is what this really meant" story and wanted to verify or discount it.
So I dug out Robert Jewett's relatively recent Romans commentary, quickly on its way to become the commentary on my list of Ken's Picks, although I want to use it a little more before committing. (By the way, I wrote too much on my second Paul book so Wesleyan Publishing House is probably going to split it in two, making the next volume, Paul: Soldier of Peace, entirely on Romans.)
So here's the situation as best I can figure it out. First, Paul is quoting Proverbs 25:22. Since Proverbs draws on some Egyptian literature, it is likely that it alludes to an Egyptian practice of a repentant person carrying a thing with hot coals in it on their heads to symbolize their repentance. The idea of the Proverb would thus be to feed and give drink to your enemy so that he or she will repent of their animosity toward you.
Of course Paul is unlikely to know the Egyptian background of the Proverb, which makes that information irrelevant for interpreting Romans (this is a key point of learning that many "scholars" do not even get. Doing a Hebrew word study or contextual exegesis of an OT passage is irrelevant to the interpretation of a NT passage that engages the OT if the NT author isn't likely to have known such things. By and large, the NT authors used the Greek OT and didn't read the OT using historical methods).
So there may have been interpretive traditions around that Paul had in mind, ones that we apparently have no information about. On the other hand, Paul may have been in the same boat we are as far as reading the verse in context. We can tell the context is about something positive, not negative. We can tell you are doing something good for your enemy, and it is moving them toward reconciliation with you.
And this is exactly the meaning Paul sees in Romans 12:20 as the Spirit leads him to write to the Romans. It is possible, although we cannot really say, that the expression was just as much a dead metaphor for Paul at the time as it is for us. We pretty much get what it is saying without really knowing where it came from.