I don't understand, why recognize the grandmothers at all?
Not to sound like an ass, but the guests know who they grandmothers are, why do something to point it out? It's all about the MTB, not the GMTB, as far as I'm concerned. My mom and MIL were congratulated by guests at my shower, they didn't need any special recognition...
I'm picturing my mom and MIL walking around in those awful sashes...not that they'd ever do that.