Can you make the sashing 1/2 the size you want and add it too all the blocks?
In order to put the sashing around every other block, 1/2 of the blocks need to be made larger - example finished block size 12" sashing 1" so 1/2 the blocks need to be 10" and to these you would add the sashing leaving the rest plain. That way when you add " them together all your blocks are 12"
Clear as mud?
Not sure you can do this with a D9P without losing the pattern of the quilt.