Unless you’ve read some official Oracle document that said it should work, it sounds like they have finally broken the backward compatibility. Perhaps you should consider that you’ve gotten your money’s worth, at this point, and it’s time to upgrade? Seems odd that they’d break it on a minor revision, but it might have been unavoidable or considered unimportant, given the timespan.
But, you could always ask the folks at Oracle. Regular support of 6i ended a year ago, but “Error Correction Support” extends to the end of this month (Jan. ’05). If it’s supposed to work, that should be covered under Error Correction.