If there is no Classic OS ROM available (or written for a particular Mac model) I don't think much is going to happen.

The FW800 MDDs are peculiar beasts in that the later 2003 FW400 models did work with OS 9. I'm guessing that the FW800 are different beasts indeed, perhaps close to the single 1.6Ghz G5 tower in terms of components used.

JB