If the morph could contain a symbol for forced non-promotion (which, unlike an indicated no-op, would then suppress any promotion specified in the captureMatrix), you could simply use that to suppress the capture-driven promotion on an arbitrary part of the board.
Perhaps this can just be a forced no-op, as a non-promotion would just leave the piece as is.
Also, having the forced Shogi promotion and promotion with choice parameters available in captureMatrix would be nice as well. However, you would need to use a different symbol for the forced Shogi promotion there, since ^ is already used for hopping restrictions (I think the # sign would be a good choice).
Perhaps this can just be a forced no-op, as a non-promotion would just leave the piece as is.
Also, having the forced Shogi promotion and promotion with choice parameters available in captureMatrix would be nice as well. However, you would need to use a different symbol for the forced Shogi promotion there, since ^ is already used for hopping restrictions (I think the # sign would be a good choice).