it seems the original definition of the word cameo is linked to the shell carvings, and the great cameo artists did of course 'carve'. Maybe the thinking is that......... provided you are 'cutting' back (to create a design) of one colour which has been overlaid completely on another, then you still end up with the same result, which is.............a design in relief (whether the process be dry or wet). There are similar effects produced in gem stones, although that is a dry process.
It seems there were slightly varying methods of creating the final appearance of this 'pseudo cameo' effect. If you look in Roger Dodsworth and Hajdamach, there are examples of both E. & L. and Richardson pieces, which are created using slightly different techniques. Pieces from those factories also apparently carry the appropriate backstamps, and it's easy to see why these two factories made use of the process..............since they were both owned by Thomas Webb.
I have a Celtic sort of design on a Webb plate, with blue cut back to a clear hammered finish, although I had never thought of it as pseudo cameo - I'll post a pic tomorrow.