I don't think this practice is very widespread, due largely to government
inspections of seafood processing plants, but it has been done.  Unlike
"artificial crab" or "sea legs", which are openly offered as a less
expensive alternative to real crab meat, these "artificial scallops" have
not been presented as artificial.  Rather, attempts have been made to pass
them off as the real thing.  Consequently, you are not likely to find any
indication of their true nature on a menu.  In a scallop, all that is eaten
is the large adductor muscle which holds the two valves of the shell
together (unlike clams or oysters, where you can eat everything but the
shell).  This makes them easy to counterfeit.  All you need is a cylindrical
punch of the right diameter and you can punch out "scallops" from any
light-colored muscular tissue.  A favorite source of such "scallops" has
been the wide lateral "wings" of skates and stingrays.  Once you remove the
skin, you have a large, flat slab of whitish muscle, well suited for
"scallop" punching.
Paul M.