I think it's definitely more of an internet niche. But plenty of dealers like Metropolis, Dale, Bob Storms, Comiclink, etc. are well aware of the niche and often mention when a book is a Picture Frame on their websites. Though I don't think they'd make a special sign about it for a show. If they got a new group of the books, they'd probably just contact some of us PF fans and let us know about it.
Speaking of which, Barton.....
Just picked up a nice new Bronze Age/ Copper Age collection and there are many picture frames in it, including Westerns and Romance books. You might stop by if you are coming to SDCC. Be good to see you.