the advantage to using a flat batting (eg., warm and natural or w & white) instead of a flannel is that the batting will let the seam allowances sink in and the top will not be a "bumpy" ...but stability of cups and glasses will still be a problem. One way to "piece" a block is to just overlap the seams and use decorative stitches to do the quilting and stitch the pieces together at the same time.In this case, a flannel center would be fine since there would not be the traditional bulk of seam allowances. A single, very large block, might be practical, especially if you use something like a star where the covering can be adjusted so that individual places for eaters can be positioned between the star blocks....or how about appliqueing placemats to the perimeter with only the very center as an actual block...