Seeing as most people today don't believe in the supernatural; or at least don't believe in most of the older supernatural pantheons, the most likely people to seek out these kinds of deals would have been war lords of ancient times up until several hundred years ago.
Take for example the Merovingian King Clovis I; who is historically important because while conquering Gaul in the 6th century, he converted to Nicene Christianity and doing so got the Frank tribes he led to do the same thing. If you read his conversion story, it is not inspirational or have any feeling of being a miracle. It reads like someone who took the Pepsi challenge and decided that Pepsi does taste better than Coca-Cola.
Clovis was in a battle with a rival and was losing and is reported by his biographer Gregory of Tours to have said something like this: "Jesus, I know of you." (Clovis wife Clotilda had already converted and Clovis wasn't happy about that. She was the only person who would stand up to Clovis that he would not kill, because he was known for killing anyone else who did so.) "My gods are letting me down. If you can do better, I will follow you and make my people do the same." (I'm heavily paraphrasing here.) Right about then an arrow or spear hit the enemy leader in the head and killed him outright, leading to an instant victory for Clovis as the enemy forces; seeing their leader fall, fled the field.
In a Dresden Files universe you could easily see someone like Clovis; instead of being converted to Christianity (Sort of, he was still an unpredictable and extremely dangerous man who killed most of his family, except his wife and children.) making a deal with Odin or any other supernatural heavy hitter who has an interest in humanity.
The thing is, there have been many, many kings and rulers and a lot more petty warlords who aspired rule their own kingdom. So, the real question is, why would Odin or any supernatural being like Mab want to back any particular mortal? It would have to be someone who could push the agenda of that supernatural being and be in the right place and time to do so.
So, maybe not very many rulers or would be rulers would have had the potential that Mab, Odin or any other big supernatural player, would have wanted, to make the kind of difference they desired. A lot of those who tried to make deals might have ended like the trumpet player in Summer Knight, who told Maeve that he would die in order to play a great solo. Remember, he played the solo and he then died. These type of leaders and wouldbe leaders would have just been play things for much of the supernatural world.
