Ok, here is my solution... first remove the default implementation of SearchText and then add our.

SearchClient.Instance.Conventions.ForInstancesOf<PageData>()
    .ExcludeField(page => page.SearchText()) // Exclude the default SearchText
    .IncludeField(page => page.SearchText(true)); // Include our extened SearchText

And the actual implementation of our SearchText

public static string SearchText(this PageData page, bool extended)
{
    StringBuilder content = new StringBuilder();

    // Removes dynamic content plugins
    string text = Regex.Replace(page.SearchText(), @"\{\w+\}", string.Empty);

    content.AppendLine(text);

    // Add extra content from "tab pages"
    foreach (var tab in DataFactory.Instance.GetChildren(page.PageLink))
    {
        if (tab.PageTypeID == Settings.Instance.TabPageTypeID)
        {
            content.AppendLine(tab["TabHeading"] as string);
            content.AppendLine(tab["TabBody"] as string);
        }
    }

    return content.ToString().StripHtml();
}