Potresti risolvere inserendo un attributo nel tag ad esempio:

Codice PHP:
<tag>
    <name>select</name>
    <tagclass>percorso.del.tuoPackage.SelectTag</tagclass>
    <bodycontent>empty</bodycontent>
    <info>Crea una select</info>
    <attribute>
        <name>nome</name>
        <required>true</required>
    </attribute>
    <attribute>
        <name>list</name>
        <required>true</required>
        <rtexprvalue>true</rtexprvalue>
    </attribute>
</tag> 
La tua classe Tag

Codice PHP:
public class SelectTag extends TagSupport {
    
    //...
    
    public int doStartTag() {
        try {
            JspWriter out = pageContext.getOut();
            out.println("<select name=\""+nome+"\" >");
        
            if (list!=null){
                Iterator it=list.iterator();
                while(it.hasNext()){
                    Selectable s=(Selectable)it.next();
                    out.println("<option value=\""+s.getValueID()+"\">"+s.getValueName()+"</option>");
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
        
        return TagSupport.SKIP_BODY;
    }
    
    public int doEndTag(){
        try {
            JspWriter out = pageContext.getOut();
            out.println("</select>");
        } catch (Exception e){
            e.printStackTrace();
        }
        
        return TagSupport.EVAL_PAGE;
    }
} 



public interface Selectable {
    public String getValueName();
    public String getValueID();
} 

E infine la jsp:

Codice PHP:
<jsp:useBean id="tuoBean" scope="......." class="percorso.delPackage.TuoBean" />
<%@ taglib uri="/WEB-INF/selectTag.tld" prefix="st" %>
<html>
<head>
    <title>Custom Tag</title>
</head>
<body>
      

<st:select nome="s" list="<%=tuoBean.getLista()%>"/></p>
</body>
</html>