Hi Adam,

If you have an umbraco.tv subscription, there is a tutorial on "Modifying the backend UI with page events" which gives a basic example of adding buttons to the toolbar.  Basically you need to attach to the umbracoPage.Load event and insert the buttons.  Below is some basic code I have used to do this in the past:

        void  umbracoPage_Load(object sender, EventArgs e)
        {
            umbracoPage page = (umbracoPage)sender;

            // Temp fix for some alignment issues 
            string strCss = "<style type='text/css'>.mceToolbarExternal{left:108px;}</style>";
            page.Page.ClientScript.RegisterClientScriptBlock(this.GetType(), "cssfixtoolbar", strCss);

            if (page.Page.Request.Path.ToLower().Replace((GlobalSettings.Path + "/").ToLower(), "").Contains("editcontent.aspx")) 
            {
                Control control = ((ContentPlaceHolder) page.FindControl("body")).FindControl("TabView1");
                if (control != null)
                {
                    int nodeId = int.Parse(HttpContext.Current.Request.QueryString["id"]);

                    TabView view = (TabView) control;
                    foreach (TabPage page3 in view.GetPanels()) 
                    {
                        MenuIconI ni = page3.Menu.NewIcon(4);
                        ni.AltText = "View Page in Flash";
                        ni.OnClickCommand = string.Concat(new object[] { "window.open('/?start=", nodeId, "&mode=flash&isPreview=1','flashPreview')" });
                        ni.ImageURL = "/media/assets/icons/preview-flash.png";

                        MenuIconI ni2 = page3.Menu.NewIcon(5);
                        ni2.AltText = "View Page in Text View";
                        ni2.OnClickCommand = string.Concat(new object[] { "window.open('", umbraco.library.NiceUrl(nodeId),"?isPreview=1','textPreview')" });
                        ni2.ImageURL = "/media/assets/icons/preview-text.png";
                    }
                }
            }
        }

Hope this helps,
Tom