I think that using a Mamiya lens is a stroke of genius for doing DIY tilt-shift lenses, mainly for two reasons: for one those lenses can be found on eBay for around 50-150 US Dollars and they provide superior quality for the price.
The second reason has to do with the optic qualities of large format lenses. A large format lens has to cover a large piece of film (or a large piece of sensor), as a result it casts a large image onto the film plane. This allows light to hit the sensor even if the image is tilted or shifted. But it gets better, the Flange Focal Distance – the distance a lens requires from its rear end to the film plane – is larger for medium format cameras so using a Mamiya lens allows having some bellows between the lens and body while still allowing non-macro photographs to be taken.
You can read the specifics on the build on digital camera world, until then, here is a short movie that shows what this camera/lens combo is capable of:
[via digitalcameraworld, thanks for the heads up, Peter]