If you add another opening in the rail, you will also need to add a stanchion, so that you get support at both ends of the opening.
A better idea might be to board from the transom, where you already have an opening and steps. If your boarding is from the Satrboard side, you should relocate the bottom transom step to the Starboard side of the transom, then you have an easy step from the dock to the lower step. without having to go to the middle of the swimgrid first.
On my C&L 44 (a MT lookalike) I relocated that step years ago and have never regretted it.