I like the idea of a jeans border, maybe also cornerstones. I have not tried either or your full sashing plan, my concern is the varying thicknesses. As far as the T-shirts, the deal is that they stretch and you don't want that. I use a light weight fusible and apply it BEFORE cutting it out (I do slice up the sides and neck but a lot bigger than I want the finished block to account for the 'curl' of the material). I also like to do an echo stitch around whatever the logo is with just the T-shirt and the interfacing. For me, it just helps hold everything together and I have no worries about it after many washes. I like to do a minimal quilting on my tops so this doesn't clutter up the backing. For quilting, I usually do something meandering down the sashings and a stitch inside the block about an inch or so inside. A thought - do you have some of his 'regular' shirts? Could you use the cotton shirts as sashing and border with the jeans?