I can't find anything that addresses this, and since my brick-sized Gameboy is long gone, I can't check myself, but logic would indicate that a Gen II shiny Pokemon cannot be shiny in Gen I because shinies didn't exist in Gen I.
Additionally, since like you said shiny Pokemon were determined by their IVs, if you transferred the shiny Pokemon back into Gen II, it should once again be shiny, because it'll have the same IVs.