I don't think it a bad idea. At the very least, a test run to see whether it motivates duellists to get out there and challenge.
I do think, however, that a simple challenge league isn't enough. People can easily abuse it by challenging friends and retracting the challenges. Perhaps make it so that the system is perhaps based more on kills rather than challenges. Since the only way a duellist can kill someone is in a challenge or duel, it amounts to the same thing, yet insures the system isn't being taken advantage of.
Written by my hand on the 21st of Midsummer, in the year 1211.