I actually saw one(a sign outside) in my area a few years ago. The inspector had done a listing inspection and proudly displayed it in the house. He also said he was an ASHI member and displayed the logo.

Couple problems:
1. He wasn't an ASHI member.
2. He didn't identify a few things that I felt were important for my client..
if I remember right...
a. FPE panel that had a bunch of problems
b. a disconnect for a heat pump that had scorch marks on the outside and melted stuff on the inside
c. some structural stuff with the foundation walls
d. plumbing that had classic copper/galvanized connections with the related blisters, bubbles and rust on them, and leaks too if I remember.

Oh I forgot one other thing, he was also a Realtor in the same office as the listing agent.

I really don't see the benefit of putting out signs.
JF