Many years ago when Pokemon Red / Blue was immensely popular, I attended a Pokemon tour in Melbourne. For attending you got a showbag with a Pikachu trading card (with a gold logo stamped in the top right corner), a book with Episode 1 of the TV show in a comic-book format, some stickers / temporary tattoos and if you brought your game cart along, you could get Mew using some specialised hardware they had.
From recollection (this was 15+ years ago now. Yikes!) the hardware ran a modified version of Pokemon red or blue (not sure which). You'd make a slot (first or last, can't remember) empty in your party, pop your cart in the top of the machine and they'd whack a button on the control pad (a SNES controller, I think?). Mew would transfer over, before the game would "lock up" and revert back to the initial "Press a button to transfer" screen. I don't remember if the revert was automatic or if the attendant had to press a reset button, but it was a rather quick and easy procedure.
I suspect this hardware maybe used two Super Gameboy 2's (which had a Game Link port, where the original SGB didn't) with a modified GB cart in one, and the other SGB2 modified so you could hot-swap carts so you didn't need to get people to head to Pokemon centers, hook up, start trading etc.
Does anyone have any more information about this hardware? I've got a keen interest in modified Nintendo Hardware (devkits, demo units etc.) and would love to know more.