RTL and MathJax

Yes, that would be difficult. It’s not an ideal solution, but you can add a direction toggle to the composer with a theme component. That will allow you to switch the direction of the entire composer edit area to ltr when you are writing code. The code below is extracted from the code that is used when the support mixed text direction site setting is enabled. If your site’s default direction is rtl, keep the code as it is. If the default direction is ltr, replace 'rtl' in the ternary operator with 'ltr' to avoid having to double click the first time the toggle is used.

<script type="text/discourse-plugin" version="0.8.32">
    api.onToolbarCreate(toolbar => {
        toolbar.addButton({
            id: "toggle-direction",
            group: "extras",
            icon: "exchange-alt",
            shortcut: "Shift+6",
            title: "Toggle Direction",
            perform: () => {
                const $textArea = $(".d-editor-input");
                let currentDir = $textArea.attr("dir") ? $textArea.attr("dir") : 'rtl', // set the default to your site's default text direction.
                    newDir = currentDir === "ltr" ? "rtl" : "ltr";
                $textArea.attr("dir", newDir).focus();
            }
        });
    });
</script>

Click the Toggle Direction icon to change the composer’s direction: