Mmmmh... Maybe not a "peaceful" leader, but a "builder" leader : being at peace 90 turns (540/6) is pretty easy, and in most games, almost every Civ will get a leader after the first 90 turns. Maybe we should have other events to trigger the appearence of a leader non resulting from war : building a certain wonder, earning some number of culture points in total, having some per-turn culture points in one city, discovering (not trading) a certain difficult tech etc.
Varying the triggers would probably add some depht to the builders strategy, and having leaders simply for peace could be a good idea. But earning a new leader simply after 90 turns is not the good calculation, as many civs will get one at the same time. Maybe something else, like being x+x/2 turns at peace in a row, where x is the number of turns the second most peaceful Civ was at peace in a row at its best... Wow, I don't think I was clear, here's an example : the Indians are the second most peaceful Civ you know, having waged peace 40 turns in a row ; when you have waged peace 60 turns (40 + 50% of 40), you get a peaceful leader.
Just ideas, but I still think leaders for peacemongers should be less numerous than leaders for warmongers, since peacemongers would be then tremendously advantaged to build wonders. Any suggestions ?