Here's my 2nd attempt: 

public static string GetContent(string content, int pageId = 0)
{
    var regexMacro =newRegex(@"<\?UMBRACO_MACRO[^>]+>");
    return regexMacro.Replace(content, delegate(Match m)
        {
            var aliasMatch = Regex.Match(m.Value, "alias=.([^\"']+).", RegexOptions.IgnoreCase);
            if (!aliasMatch.Success) return string.Empty; // macro must be screwed up to not have an alias

            var macro = umbraco.macro.GetMacro(aliasMatch.Groups[1].Value);
            var attribs = new Hashtable();
            macro.Model.Properties.ForEach(p =>
                {
                    var pMatch = Regex.Match(m.Value, string.Format("{0}=\"([^\"]+)\"", p.Key),
                                             RegexOptions.IgnoreCase);
                    attribs.Add(p.Key, (pMatch.Success) ? pMatch.Groups[1].Value : string.Empty);
                });
            macro.UpdateMacroModel(attribs);
            var macroContentControl = macro.renderMacro(attribs, pageId);
            var page = new Page();
            page.Controls.Add(macroContentControl);
            var sw = new StringWriter();
            using (var htw = new HtmlTextWriter(sw))
            {
                page.RenderControl(htw);
            }
            var macroContent = sw.ToString();

            // Check to see if the macro content has another macro to render
            if (regexMacro.IsMatch(macroContent))
            {
                macroContent = GetContent(macroContent, pageId);
            }
            return macroContent;
        });
}

I received an error about not having an HttpContext so I created a dummy one in the GatheringNodeData event like so:

// mock up a context if necessary
if (HttpContext.Current == null)
{
    HttpContext.Current = new HttpContext(
        new HttpRequest("", "http://mydomain.com", ""),
        new HttpResponse(new StringWriter()))
        {
            User = new GenericPrincipal(
                new GenericIdentity("admin"),
                new string[0]
                )
        };
}

This didn't work either, so I gave up and manually gather the content based on the macro parameters. I only have two macros I use in the editor, so this is what I ended up with:

public static string GetContent(string content, int pageId = 0)
{
    var regexMacro =newRegex(@"<\?UMBRACO_MACRO[^>]+>");
    return regexMacro.Replace(content, delegate(Match m)
        {
            var aliasMatch = Regex.Match(m.Value, "alias=.([^\"']+).", RegexOptions.IgnoreCase);
            if (!aliasMatch.Success) return string.Empty; // macro must be screwed up to not have an alias

            var macro = umbraco.macro.GetMacro(aliasMatch.Groups[1].Value);

            var cs = SearchService.Instance.ContentService;
            var sb = new StringBuilder();
            switch (macro.Alias)
            {
                case "GuideForms":
                    macro.Model.Properties.ForEach(p =>
                        {
                            var val = Regex.Match(m.Value,
                                                  string.Format("{0}=.([^\"']+).", p.Key),
                                                  RegexOptions.IgnoreCase);
                            if (val.Success && !string.IsNullOrWhiteSpace(val.Groups[1].Value))
                            {
                                if (p.Key.StartsWith("formPick", StringComparison.InvariantCultureIgnoreCase))
                                {
                                    var form = new DynamicNode(val.Groups[1].Value);
                                    var formName = (form.HasValue("formName"))
                                                       ? form.GetPropertyValue("formName")
                                                       : form.Name;
                                    sb.AppendFormat("{0} {1} ", form.GetPropertyValue("formID"),
                                                    formName);
                                }
                            }
                            if (p.Key.StartsWith("formInfo"))
                            {
                                var info = Regex.Match(m.Value,
                                                       string.Format("{0}=.([^\"']+).", p.Key),
                                                       RegexOptions.IgnoreCase);
                                if (info.Success && string.IsNullOrWhiteSpace(info.Groups[1].Value))
                                {
                                    sb.AppendFormat("{0} ", info.Groups[1].Value);
                                }
                            }
                        });
                    break;
                case "GuideNote":
                    var nid = Regex.Match(m.Value, "nodeId=.([^\"']+).", RegexOptions.IgnoreCase);
                    if (nid.Success && !string.IsNullOrWhiteSpace(nid.Groups[1].Value))
                    {
                        var note = new DynamicNode(nid.Groups[1].Value);
                        sb.AppendFormat("{0} ", note.GetPropertyValue("content"));

                    }
                    break;
            }
            var macroContent = sb.ToString();

            // Check to see if the macro content has another macro to render
            if (regexMacro.IsMatch(macroContent))
            {
                macroContent = GetContent(macroContent, pageId);
            }
            return macroContent;
        });
}

If someone has a solution to either of the first two that would be great! This gets the job done at least.  Thought I'd post it for someone else travelling down this path or correct me if there's a simpler path.

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