If you're willing to forgo 29" for 26"

If you find the right frame (for instance, this 1984 Trek something 531 frame I have), you can get lucky. I discovered almost by accident (I was tired, and trying to true my MTB's wheels, and put one in the trek frame), that it will take a 26"x2" in the front, and a 26x1.75" in the back, if there aren't extreme side knobbies in the back.

Brakes would be an issue, but you could fashion something that would work, I'm sure.