I looked for a while on how to transform pipe-delimited values from a field list-box and couldn't find anything specific - only general information about how to write a transformation method. So, I did that and tried to make it pretty generic and thought I'd share what I came up with. If anyone has any suggestions for improvement, they would be greatly appreciated.

public static string Lookup(object _value, string _classNames, string _valueField, string _textField)
{
    NodeSelectionParameters _params = new NodeSelectionParameters();
    _params.SelectOnlyPublished = true;
    _params.SiteName = CMS.CMSHelper.CMSContext.CurrentSiteName;
    _params.ClassNames = _classNames;
    _params.Where = _valueField + " IN (" + _value.ToString().Replace('|',',') + ")";
    _params.OrderBy = _valueField;

    TreeProvider _tree = new TreeProvider();

    var _dataSet = _tree.SelectNodes(_params);
    if (_dataSet != null)
    {
        StringWriter stringWriter = new StringWriter();
        using (HtmlTextWriter writer = new HtmlTextWriter(stringWriter))
        {
            var _nodes = _dataSet.Items;
            if(_nodes.Count == 1){
                writer.Write(ValidationHelper.GetString(_nodes[0][_textField], String.Empty));
            } else {
                writer.RenderBeginTag(HtmlTextWriterTag.Ul);
                foreach (CMS.DocumentEngine.TreeNode _node in _nodes)
                {
                    writer.RenderBeginTag(HtmlTextWriterTag.Li);
                    writer.Write(ValidationHelper.GetString(_node[_textField], String.Empty));
                    writer.RenderEndTag();
                }
                writer.RenderEndTag();
            }
        }
        return stringWriter.ToString();
    }
            
    return "";
}