I know practically NOTHING about heirloom fabrics or satins et.al. but I did have 2 favorite receiving blankets for my twins which we used at their christening (as well as all summer) they were a very lightweight cotton... almost a batiste maybe?... very plain (light blue) w/ a tiny lace edging and their names embroidered on the corner. Sorry I'm not remembering your grandson's age, but if this is an infant baptism, something very plain might be perfect-- it lets HIM be the star of the show
But in order to use the satin, maybe just a border, as a heavier fabric would give a lightweight fabric some "body". If it's a heavy taffeta, then piecing it in any way might make it TOO heavy?
Just a thought... Florence