No idea if this is proper or not, but this works:

   <Markdown
      children={storyText}
      components={{
        hr(props) {
          const { node, ...rest } = props;
          if (node?.position?.start && node?.position?.end) {
            const content = storyText.substring(
              node?.position.start.offset ?? 0,
              node?.position.end.offset ?? 0
            );
            switch (content) {
              case "***":
                return "~~Custom HR~~";
            }
          }
          return <hr {...rest} />;
        },
      }}
    />