I would use the DepthOfFieldFunction material expression in your material to approximate depth-of-field yourself. That node generates a value of 0 when the pixel should be completely in focus and a value of 1 when it should be out of focus.
The simplest thing you can do there is to mip bias your texture lookups making them blurrier. You would do that by setting the MipValueMode of the texture sample in the material editor to “MipBias” and then connecting the result of depth of field function to the MipValue input pin. You can multiply up the result of the DepthOfFieldFunction to control how blurry the texture is when out of focus.
You could also fade between an in-focus texture and out-of-focus texture that has been pre-blurred in an application like Photoshop. That will let you use a nicer filter for blurring the texture than the box filter used to generate mipmaps.
More info:
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After writing this up I did a quick test and noticed that the DepthOfFieldFunction expression is not working correctly! I’ve filed bug UE-7950 to get that fixed up.
In the meantime, you can use the PixelDepth node to fade between in-focus and out-of-focus depths to blur your textures. If you set your in-focus and out-of-focus depths using a Material Parameter Collection you can access them easily from any material.