Perhaps this can just be a forced no-op, as a non-promotion would just leave the piece as is.
I already made it such that forced non-promotion in the morph can be achieved by specifying promoting to the piece type itself. Before this was excluded, to allow two pieces using the same ID (and presumably the same image) to morph into each other (to emulate a location-dependent move). By excluding the self it would then automatically pick the other. But there wasn't any use in also forbidding it if there only was a single piece with that ID. So this is now allowed.
Forced Shogi promotion is really redundant: you can just mention the oppromoted type. The possibility to use comma separation in a row of the matrix allows this even when the ID of the promoted type is something like +L.
I already made it such that forced non-promotion in the morph can be achieved by specifying promoting to the piece type itself. Before this was excluded, to allow two pieces using the same ID (and presumably the same image) to morph into each other (to emulate a location-dependent move). By excluding the self it would then automatically pick the other. But there wasn't any use in also forbidding it if there only was a single piece with that ID. So this is now allowed.
Forced Shogi promotion is really redundant: you can just mention the oppromoted type. The possibility to use comma separation in a row of the matrix allows this even when the ID of the promoted type is something like +L.