is there anyway a shader could have a watermark

[steven tyler sean herr]

as the title says is there anyway to have a shader have a transparent watermark?
the reasoning being
i want to see gba games that look like it was given the four swords adventures treatment

fsa used a texture to make two tiles that would otherwise look the same (eg two solid green grass tiles)
look diffrent

i want to do something similar but since i couldnt apply a static texture on always loading and unloading tiles
i figured a watermark with a transparent texture would be second best