I think there are lots of places that have a type of living history type setup (think Williamsburg in VA), but they are for the tourists. I have never heard of a group where they live in the middle ages full time, rather it is more of a day job. Do you have a region or a place name you could give us and we can help you?
-Lastly, If such a place does exsist, are you sure that they take people off the street, and how good is your Japanese? Even for WIlliamsburg there is a very long process you go through before they let you out in costume.
I think you idea sounds like a lot of fun though!