Well, having no suggestions yet, I made some myself:
All castlings placing the King three squares from the left white side will be written as O-O-O (standard).
All castlings placing the King two squares from the right white side will be written as O-O (standard).
All other castlings will be written e.g. O-O-b terminated by the King's target column letter, here 'b'.
Would that be acceptable?