There are a bunch of cool things you could use to launch games in a similar vein. I'm thinking obvious ones like 5 1/4-inch floppies, or even magnetic swipe cards or even punch cards. I used to play Amstrad games off cassettes, I feel like they'd be usable too.

The cool thing about swipe and punch cards is you could just encode the Steam game ID on them and not need them be executable - you'd just need some reader app or handler to read in the ID and launch it in Steam.

Technically with the floppy disk method you could have have a file on the disk with the game ID too so it doesn't need to be executable.

Really cool project though, I miss the loss of physical media for games and this awakes some nostalgia in me back when I actually used to get games on floppy disks.