Perhaps you misunderstood the skinning docment. The <![CDATA[]] /> parameter is only used in the skin.xml file, and the parsing engine uses this to build the correct string in the .ascx file.
The .ascx code you want is:  <dnn:SOLPARTMENU runat="server" separator="<%= SkinPath %>separator.gif" />

I may have steered you wrong, and for that I'm sorry.  I'll give you two examples of SolPartMenu separators here that absolutely work, and were done by me just this week:
First one:
.ascx:
<dnn:SOLPARTMENU runat="server" id="dnnSOLPARTMENU" Display="Horizontal" SeparateCss="True" rootmenuitemrighthtml="&nbsp;" separator="&lt;img src=&quot;images/menuSep.jpg&quot;&gt;" separatorcssclass="MenuSep" />
.css:
.MenuSep {
background-image:url(menuSep.jpg);
padding:0px;
width: 5px;
}
.MainMenu_MenuBreak {
    height: 4px;
}

Second one:
.ascx:
<dnn:SOLPARTMENU runat="server" id="dnnSOLPARTMENU" Display="Horizontal" SeparateCss="True" submenuitembreadcrumbcssclass="submenuitemselected" submenuitemactivecssclass="submenuitemselected" submenuitemselectedcssclass="submenuitemselected" separator="&nbsp;" separatorcssclass="MenuSep" rootmenuitemlefthtml="&nbsp;" rootmenuitemrighthtml="&nbsp;&nbsp;" />
.css:
.MenuSep {
background-color: #BC2525;
background-image:url(images/menu_break.gif);
background-position: center;
background-repeat: repeat;
width: 2px;
height: 34px;
color: white;
}
.MainMenu_MenuBreak {
    height: 34px;
    border-top: 1px solid #BC2525;
    border-right:  0px solid #BC2525;
    border-bottom:  1px solid #BC2525;
    border-left:  0px solid #BC2525;
}

Hope you can use something from this.