If all you are looking for is that the user types in a value = to a value in your list... 
Use a custom validator with this as your validation function:

function ValidateCombo(oCombo,ColIndex) {
    try {  

        var grid=oCombo.getGrid();
        var rowCount=grid.Rows.length;
        var currVal='';
        var comboText=oCombo.displayValue;

        for(i=0;i<rowCount;i++){
          var row=grid.Rows.getRow(i);
          currVal=row.getCell(ColIndex).getValue();
          if (currVal == comboText) {
            return true;
          }   
        }
        return false;
    }   
    catch ( err ) {
        var txtMessage = document.title + " - Error in ValidateCombo()\n" + err.description ;
        alert( txtMessage ) ;
    }  

}

You can add a hidden input element to your page, update the value of that element whenever the value of the WebCombo is changed, and apply your validator against that input element.

WebCombo cannot be directly used as the target for an ASP.NET validator.