If you just want to see the bios animation, you can just use an audio cd. It will stay in the press start screen with the animated logo.
If you want to access the cd player or backup memory manager, with an audio cd, just press start. With a game cd, just press any button except start.
Ability to properly run a sms bios has been added to the next firmware, you'll just need to load any dummy file renamed as a sms game, and you'll be able to run the internal bios games