Immunity is not a property in itself, but just the absence of contagion for all captures of that piece, even when other pieces might suffer contagion (which again is a particular form of automatic promotion on capture) when capturing some victims. It seems to me that the move-dependent immunity that you want can already be achieved by specifying promotion to the contageous victim for the capture of it in the captureMatrix, and then exempt the move that you want to have immunity through an apostrophe.
Immunity is not a property in itself, but just the absence of contagion for all captures of that piece, even when other pieces might suffer contagion (which again is a particular form of automatic promotion on capture) when capturing some victims. It seems to me that the move-dependent immunity that you want can already be achieved by specifying promotion to the contageous victim for the capture of it in the captureMatrix, and then exempt the move that you want to have immunity through an apostrophe.