it would work just fine to quilt your 6" block then make it larger, the extra loft will add more to the over-all design of the quilt. the only thing i think may be an issue is the second time around quilting...the added bulk and added backing may make it more difficult to hand quilt through, as long as you are using a good-new sharp needle it will probably not be too difficult. i would try a couple blocks...maybe 3 or 4 into a table runner to test the technique on a (start to finish) little project to see if i need to modify anything to make it work.