I received my 100-in-1 multicart, and the Magic Key converter DOES work. BUT! There is a catch. The multicart simply won't boot to the menu unless you're holding down the passthru button on the converter itself. Hold it when you power on, and you'll get to the menu just fine. Step 2, you pick your game, and you're greeted with a black screen. You can still call up the cheat menu, but the game won't start. Hit reset on your AES with the passthru button held again, and your chosen game will launch, and it'll play perfectly in AES mode! Of course this does mean you won't be able to use the Unibios in the converter itself.
Tested on a JPN unmodified AES, serial 169470. Games tested at time of post;
Samurai Shodown 5 Special
Blazing Star
Metal Slug 2
Metal Slug 3
Metal Slug X
Super Sidekicks 3: The Next Glory
The Last Blade 2
Art of Fighting 2
Shock Troopers 2: Second Squad