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Thank you, George, for explanting. In SMIRF I already have implemented traditional, symmetric and modern castling, which is defined by the King's destination squares and thus also will work for randomized setups.

But having randomized pieces, there might be Kings neighbored or near to Rooks, thus the given definiton of free castling will not be usable for such varying starting arrays, because it would ban any castling towards such blocked sides. That is a weakness, as I think. A more flexible definition therefore would be welcomed and moreover would less interfere with any claimed patents.  

How about following (which probably will be in conflict only marginally with some of those games using free castling): you can choose any square on the base row to be the King's castling target field except of the center and the border squares. The left squares are related to the left Rook, the right squares are related to the right Rook. A castling Rook will always be placed to the inner side of the castled King. Only still unmoved pieces will be allowed to castle. Castling is invalid under check or if the King will have to pass or reach a threatened square. All squares between King and its target (included) and between Rook and its target (included) have to be free from third pieces (therefore at least all squares between King and Rook have to be free).

A clear notation would be O-c-O, with a central letter related to the King's target square column.