I've seen a couple tutorials on designing Mondrian-style quilts, and they all used Excel rather than EQ. Basically, you make the cell size horizontally and vertically match the width of your black lines, then fill in the cells with your desired colours. The quilts are assembled into subunits (not really blocks) of whatever size works, then those are joined.
I looked to see if I could find a tutorial, but no luck. I know I have seen it done this way at least twice, though!