This doesn't answer your question regarding adapter, but I wanted to caution you that you probably won't capture the entire image circle: it will be cropped a bit on the top and bottom edges of the Pentax's frame.

The EF lens projects an image circle with a diameter that is at least as large as the diagonal dimension of the EF body's sensor. Specifically, the 24mm × 36mm sensor has a diagonal of 43.3mm. The image circle of the EF lens must cover this distance (plus probably a little bit extra).

The height of the Pentax's film/sensor plane is 45mm, just 1.67mm wider than the EF's minimum image circle. So there's not a lot of room for error there.

However, the flange focal distance (FFD) of the Pentax is 70.87mm. The FFD of the Canon EF mount is 44mm. Assuming your EF-to-645 mount adapter is a zero-depth adapter (which is actually quite possible, considering the substantially larger diameter of the Pentax 645 mount), then the light cone of the EF lens will project an additional 26.87mm.

You can think of the light cone of the EF lens as a cone with a base diameter (minimum) of the EF sensor (43.3mm), and with a height of the FFD plus the lens's exit pupil distance (from the lens flange). I'll call the exit pupil distance P.

So the lens is mounted an extra L millimeters further from the sensor than normal, its light cone will spread out to a size of 43.3mm × (FFDPentax + P)/(FFDCanonEF + P).

I don't know what the exit pupil distance of your lens is. But just for the sake of example, let's use 25mm. Then the light cone of your Canon EF lens mounted on the Pentax 645 would spread out to a diameter of 43.3mm * (70.87 + 25)/(44 + 25) = 60.2mm, which is just past the width of the 645's film/sensor plane.

The calculation is worse the smaller the exit pupil distance is. For a hypothetical 0mm exit pupil distance, the extended image circle would be 43.3mm * 70.87 / 44 = 69.7mm.

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Now, if you used an EF-S lens that projects a "tight" image circle for Canon's APS-C sensors (1.6x crop factor), then a 0mm exit pupil lens would project (43.3 / 1.6) * 70.87 / 44 = 43.6mm. This is less than the 645's smallest dimension, and again, is a worst-case size (with regards to exit pupil distance).

Thus, if you want to capture a full image circle without being cropped by the 645's frame size, you need to use a Canon EF-S lens, or any lens designed for a Canon APS-C sensor (or smaller, such as Micro-4/3).