there is multiple things that concern me here. one that it is a stucco finish and second that it is over an entrance. i wouldnt offer a waiver i would insist that i install it with mechanical fasteners or that the customer takes the sign and installs it themselves.
We decided to attached a substrate/base to the wall first, then apply a liquid agent that will adhere to the base.
I have a client that says absolutely no to drilling or screwing into their exterior parking garage wall. We have a 12"Hx60"W direct print dibond sign that needs to be adhered above a 15' entrance. Is there anything besides liquid nails???