Sí existe documentación del API:

"If you don't want to load all ResultSet data in memory and agree to keep database connection open while processing all data you may use net.sf.jxls.report.ResultSetCollection class. This class takes ResultSet object as a parameter and implements Collection interface to operate with underlying data. In its turn ResultSetCollection uses org.apache.commons.beanutils.ResultSetDynaClass to return retrieved data as dynamic objects."

http://jxls.sourceforge.net/reference/resultset.html

"Any database data that you wish to utilize outside the context of the current row of an open result set must be copied. For example, you could use the following code to create standalone copies of the information in a result set:

ArrayList results = new ArrayList(); // To hold copied list
ResultSetDynaClass rsdc = ...;
DynaProperty[] properties = rsdc.getDynaProperties();
BasicDynaClass bdc =
new BasicDynaClass("foo", BasicDynaBean.class,
rsdc.getDynaProperties());
Iterator rows = rsdc.iterator();
while (rows.hasNext()) {
DynaBean oldRow = (DynaBean) rows.next();
DynaBean newRow = bdc.newInstance();
PropertyUtils.copyProperties(newRow, oldRow);
results.add(newRow);
}"

http://commons.apache.org/proper/commons-beanutils/apidocs/org/apache/commons/beanutils/ResultSetDynaClass.html

Al final he tenido que descargar la librería jXLS para encontrame con:

package net.sf.jxls.report;

import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.AbstractCollection;
import java.util.Iterator;

import org.apache.commons.beanutils.ResultSetDynaClass;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

/**
* Allows JDBC ResultSet to be used with XLSTransformer
* Note! This is very basic implementation just to give an example of how this can be done.
* You may want to create your own implementation to use advanced features of some specific jdbc driver for example.
* @author Leonid Vysochyn
*/
public class ResultSetCollection extends AbstractCollection {
protected static final Log log = LogFactory.getLog(ResultSetCollection.class);

ResultSet resultSet;
ResultSetDynaClass rsDynaClass;
private boolean lowerCase = true;

private int numberOfRows;

/**
* Construct a new ResultSetCollection for the specified ResultSet.
* The property names corresponding to column names in the result set will be lower cased.
*
* @param resultSet - The result set to be wrapped
* @throws SQLException - if the metadata for this result set cannot be introspected
* @throws NullPointerException - if resultSet is null
*/
public ResultSetCollection(ResultSet resultSet) throws SQLException, NullPointerException {
this.resultSet = resultSet;
rsDynaClass = new ResultSetDynaClass(resultSet);
try {
setNumberOfRows();
} catch (SQLException e) {
log.error("Can't get number of ResultSet records. Try to pass it to a corresponding constructor", e);
throw e;
}
}

/**
* Construct a new ResultSetCollection for the specified ResultSet.
*
* @param resultSet - The result set to be wrapped
* @param lowerCase - Should property names be lower cased?
* @throws SQLException - if the metadata for this result set cannot be introspected
* @throws NullPointerException - if resultSet is null
*/
public ResultSetCollection(ResultSet resultSet, boolean lowerCase) throws SQLException, NullPointerException {
this.resultSet = resultSet;
this.lowerCase = lowerCase;
rsDynaClass = new ResultSetDynaClass(resultSet, lowerCase);
try {
setNumberOfRows();
} catch (SQLException e) {
log.error("Can't get number of ResultSet records. Try to pass it to a corresponding constructor");
throw e;
}
}

/**
* Construct a new ResultSetCollection for the specified ResultSet.
* The property names corresponding to column names in the result set will be lower cased.
*
* @param resultSet - The result set to be wrapped
* @param numberOfRows - The number of rows in result set
* @throws SQLException - if the metadata for this result set cannot be introspected
* @throws NullPointerException - if resultSet is null
*/
public ResultSetCollection(ResultSet resultSet, int numberOfRows) throws SQLException, NullPointerException {
this.resultSet = resultSet;
this.numberOfRows = numberOfRows;
rsDynaClass = new ResultSetDynaClass(resultSet, lowerCase);
}

/**
* Construct a new ResultSetCollection for the specified ResultSet.
*
* @param resultSet - The result set to be wrapped
* @param numberOfRows - The number of rows in result set
* @param lowerCase - Should property names be lower cased?
* @throws SQLException - if the metadata for this result set cannot be introspected
* @throws NullPointerException - if resultSet is null
*/
public ResultSetCollection(ResultSet resultSet, int numberOfRows, boolean lowerCase) throws SQLException, NullPointerException {
this.lowerCase = lowerCase;
this.resultSet = resultSet;
this.numberOfRows = numberOfRows;
rsDynaClass = new ResultSetDynaClass(resultSet, lowerCase);
}

protected void setNumberOfRows() throws SQLException {
if (resultSet != null) {
resultSet.last();
numberOfRows = resultSet.getRow();
resultSet.beforeFirst();
}
}

public int size() {
return numberOfRows;
}

public Iterator iterator() {
return rsDynaClass.iterator();
}

public boolean isLowerCase() {
return lowerCase;
}

}