Dreamcast games don't interest me much (with the exception of Sonic Adventure), but let's try to sum the facts up:

-the 'floating around' version you are refering to is probably the Limited Edition (LE) one (which was dumped recently by one of my pals). The reason why you might be mistaken is that Buyatari lists his SA disc as a proto, and I noticed it's probably just a LE :)

-to get a SA proto, you'd have to bug japanese programmers (testers?) or japanese previewers, which would be very hard (everybody knows the japanese)

-there's always the probability that a big collector hoards one in his basement and can't tell us.

keep trying though. good luck =)