Well, I've tried but it didn't quite work, and I'm just not sure why.
I downloaded the 3 BIOS images for the 3 regions, catted them together (doubling the end image), so it was something like:
US
JP
EU
EU
...and burned the image to a 27c4096.
I piggybacked the new chip on the original bios, and pulled the originals CE high.
On the new chip, I lifted and tied pins 38 and 39 to ground (via 15k resistors). CE tied to the board at original location.
Result: Nothing at all. I jumpered 38 and 39 to +5 and still no reaction. I can rearrange things and get the stock BIOS to work just fine, but I can't get this chip to come alive. I went back, restored the CE lines, and tied the stock BIOS's OE high (adjusting the new BIOS accordingly) and still nothing.
My understanding is that it should either worked OR brought up a screen telling me that I'm using a Mega CD with a Genesis (or something like that).
Perhaps it just doesn't like the particular EPROM that I'm using? It's this part: http://www.atmel.com/dyn/resources/p...ts/doc0311.pdf.
Any theories, ideas, or criticisms?