Just tried it with Gens, and it works like a charm! This is amazing! It only works in the PAL version though. I tried it in the US version, and it didn't work, though my US version ROM might be a bit broken. The demo for PPZ1 has Sonic moving slow at first, and after the... "3D loop", he just moves outright weirdly. Now, we need to test this on a better US version, if mine is really broken, and we need to test every single port. ...That allows for a 2nd controller, of course. Get to it, lads!
Anyway, if you're in a hurry to get debug mode, you should just use the sound test method, because having to wait for the main menu to timeout, having to cancel the intro video, having to wait for the menu again and finally getting a demo takes a lot longer than just typing it in the sound test. This is of course a preferred method, because one won't have to memorize the correct number sequence.
I'll add it to the wiki's article in a second: Cheat Codes:Sonic the Hedgehog CD