Try this trick, cut some squares of the batiks and the background fabric. Put them all in a container together, add hot water and let them sit for awhile. Pull the background fabric out and if it's still the same color as the unwashed background fabric you can use the batiks without prewashing them. After the quilt is done make sure you wash it in a top loader so there's enough water to float away the excess dye.
Since batiks are a tighter weave and they've been boiled to remove the resist they don't shrink much at all.