Another option is to give two blocks a week, and then occasionally have a "catch up" week if several participants are getting behind. These 6" blocks can go very fast, so it might seem scary but I think once you get into it, most are going to find it's not too hard. And if you are using a constant background or theme of fabric, cutting more than one at a time might be a time saver.
It would be different if we had a lot of applique or paper pieced blocks. When I did Affairs of the Heart, one 6" applique block took me about 10 hours of hand stitching, and when doing the 50 PP stars, each star was made of 8 pp sections sewn together, and that was also a lot more work.