Mega CD Region Free Bios
I haven't done this yet. I guess you can replace your bios with this one, or modify Aarakons MultiBIOS to include this one as the fourth option (replacing the Columns game).
Check out these sites for more info - ASSEMblergames and Gamesx
Those threads mention that some games don't like working on the Euro or USA BIOS, but l_oliveira has created lots of region free BIOS files, so probably best to stick to his Japanese one (IMO, it looks the nicest also!)
UPDATE - l_oliveira has said it's okay to host his BIOS files on here. He's also pointed out that pressing ABC & Up will open the tray when using the Japanese BIOS.
