I think most people gotta understand on why Lemon oil can't be used on Maple Fretboards. Or rather, it isn't a can't in this case but a rather, should not.
Understand that Maple Fretboards are usually lacquered and sealed. Meaning, no maintenance is required from our part. Note that you can apply Lemon oil on your maple fretboard but it will only be a waste of your oil since the wood is already sealed.
The reason you oil your fretboard is to keep the fretboard from drying out and crack. So to answer your worry (Which shouldn't be one in the first place), Yes you can apply oil on your fretboard but it will not slip into the maple. And even if it does, it will not kill your guitar.