OK so the Amiga signatures were just a homage to an already established practice of doing this. Where there other machines of this era that had signatures too?
As for other machines, I don't know of other signatures, but there are often Easter eggs. Commodore machines had funny stuff on their PCBs: codenames Rock Lobster, Junebug, plus the Fred/Wilma LEDs, etc. Lots of integrated circuits from several companies had artwork in unused areas of the silicon die.