I was able to hack this out but I couldn't find a way to make the tooltip disappear when the mouse leaves the button. I set the AutoPopDelay to 2 sec. to work around this, so at least it goes away after a certain amount of time. You might want to submit a feature request for a ToolTipText property to be added to the EditorButton class.

Example:
EditorButton button = new EditorButton();
this.textEditor.ButtonsRight.Add( button );
this.textEditor.CreationFilter = new CreationFilter();

private class CreationFilter : IUIElementCreationFilter
{
    void IUIElementCreationFilter.AfterCreateChildElements(UIElement parent)
    {
        ButtonUIElementBase buttonElement = parent as ButtonUIElementBase;
        if ( buttonElement != null )
        {
            ToolTipProviderUIElement tooltipElement = new ToolTipProviderUIElement( buttonElement );
            tooltipElement.Rect = buttonElement.Rect;
            buttonElement.ChildElements.Add( tooltipElement );
        }
    }

    bool IUIElementCreationFilter.BeforeCreateChildElements(UIElement parent)
    {
        return false;
    }
}

private class ToolTipProviderUIElement : UIElement, IToolTipItem
{
    public ToolTipProviderUIElement(UIElement parent) : base(parent)
    {
        this.ToolTipItem = this;
    }

    protected override void DrawBackColor(ref UIElementDrawParams drawParams){}

    protected override bool WantsInputNotification(UIElementInputType inputType, Point point)
    {
        return inputType == UIElementInputType.MouseHover;
    }

    ToolTipInfo IToolTipItem.GetToolTipInfo(Point mousePosition, UIElement element, UIElement previousToolTipElement, ToolTipInfo toolTipInfoDefault)
    {
        toolTipInfoDefault.AutoPopDelay = 2000;
        toolTipInfoDefault.ToolTipText = "it worked";
        return toolTipInfoDefault;
    }
}