What a great idea - Since I have never attemped anything like this before I will be looking for pics of completed quilt - it sounds like you have thought of everything. You will have to make sure that the little sleepers are layed flat before you stitch around them so that the front isnt any larger to prevent sagging. O might use double sided fusible interfacing and put inside of sleepers then sew down. Wont be an easy tas but it might work.
Will there be a block with the baby's name bd etc. so its a rememberance quilt?
I posted a great poem which would work great I think and then add babys name and dates. Good luck with this project
http://www.quiltingboard.com/t-118896-1.htm