You could just use O-O for everything along with the king's destination square, like O-O b1
You could. But, at that point, I would ask what value the "O-O" is bringing. Wouldn't the more standard "e1b1" also accomplish that? Although, I guess this would address the issue with (e.g., Wildebeest Chess) where the King can move a single space and still castle. Perhaps if either "O-O" or "O-O-O" is followed by the notation of a square, it would mean castling to that square (and there would be no difference between "O-O" and "O-O-O")
You could. But, at that point, I would ask what value the "O-O" is bringing. Wouldn't the more standard "e1b1" also accomplish that? Although, I guess this would address the issue with (e.g., Wildebeest Chess) where the King can move a single space and still castle. Perhaps if either "O-O" or "O-O-O" is followed by the notation of a square, it would mean castling to that square (and there would be no difference between "O-O" and "O-O-O")