datagridhack.cshtml

<div class="datagrid_hack"></div>
<script>
    (function ($) {
        // check so that this code only fires once.
        if ($._datagridHack) {
            return;
        }

        $._datagridHack = function () {
            var root = $('.datagrid_hack');
            root.parent().parent().hide(); // hide control

            // only fire if the edit dialog box is open
            var dialog = root.parents('.ui-dialog:visible');

            if (dialog.length == 0) {
                return;
            }

            // force the dimensions of that dialog box
            dialog.css({
                width: 740
            }).offset({
                left: 0,
                top: 100
            })

            // then initialise tinyMCE on textboxes (might be better if it was more targeted)
            tinyMCE.init({
                mode: 'textareas',
                theme: 'advanced',
                width: '700',
                height: '500',

                relative_urls: false, // urls get mangled by tinyMCE defaults.
                
                // just adding some plugins that I needed. Check tinyMCE v3 documentation.
                plugins: "table",
                theme_advanced_buttons3_add: "tablecontrols",
            });
        };

        var register = function () {
            // only call our function once dom and script is ready
            $(function () {
                $._datagridHack();
            })
        }

        // check if tinyMCE lib is loaded, else force load it with jQuery.
        if (typeof(tinyMCE) !== 'undefined') {
            register();
        } else {
            var url = '/umbraco/plugins/tinymce3/tinymce3tinymceCompress.aspx?rnd=12345&module=gzipmodule&themes=umbraco&plugins=contextmenu,umbracomacro,umbracoembed,noneditable,inlinepopups,table,advlink,spellchecker,paste,umbracoimg,umbracocss,umbracopaste,umbracolink,umbracocontextmenu&languages=en';
            $.getScript(url, register);
        }
    })(jQuery);
</script>

Usage

• you create a new datatype (uComponents: Macro Renderer)
• modify DataTypeGrid.config to allow this data type.
• Add that to the datatype grid as a extra parameter.
• add a Textstring Multiple where you want a TinyMCE.

Added a check to see if TinyMCE is loaded (it is loaded if another data item has Richtext Editor, but if there isn't then Umbraco doesn't load it).