Never really got a response and couldn't find a solution either. So, please, if you know something, let me know.

What I figured out is how you can just replace the Adapter in your code like this:

protected override void OnPreInit(EventArgs e)
{
    HttpContext.Current.Request.Browser.Adapters["System.Web.UI.WebControls.Menu, System.Web, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a"] = "Supertext.Adapters.iUiMenuAdapter";
    base.OnPreInit(e);
}


protected override void OnUnload(EventArgs e)
{
    base.OnUnload(e);


    HttpContext.Current.Request.Browser.Adapters["System.Web.UI.WebControls.Menu, System.Web, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a"] = "CSSFriendly.MenuAdapter";
}

More on my blog post about this topic. Hope it helps someone.