Is it possible to place clusters on the main page instead of areas that designers should allow editing? Since the master pages are editable only in Visual Studio, this may be your only option at this point in time. One of the problems with this approach is that content placed in placeholders is unlikely to be valid, as you are likely to have tags left open in one placeholder and closed in another.
<html>
<head>
<title></title>
<asp:PlaceHolder ID="headerContent" runat="server"></asp:PlaceHolder>
</head>
<body>
<asp:PlaceHolder ID="beforeContent" runat="server"></asp:PlaceHolder>
<asp:PlaceHolder ID="centralContent" runat="server"></asp:PlaceHolder>
<asp:PlaceHolder ID="afterContent" runat="server"></asp:PlaceHolder>
</body>
</html>
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EDIT: . PlaceHolder <div>, . , PlaceHolder , HTML.