Thanks for the info. It turns out that there are jar files available specifically for connecting to an AS400. There is a project called JTOpen at http://jt400.sourceforge.net/ which deals with JDBC and systems running IBM i (or i5/OS or OS/400). I downloaded the JTOpen archive and then used the plugin wizard in Designer to create an update site.
For anyone reading this who wants to do this themselves, the class in the plugin wizard is com.ibm.as400.access.AS400JDBCDriver and the jar to include is jt400.jar. The connection file (which I called as400.jdbc and is based on John Jardin's tutorial) looks like this:
<jdbc>
<driver>com.ibm.as400.access.AS400JDBCDriver</driver>
<url>jdbc:as400://XXX.XX.XX.XX</url>
<user>username</user>
<password>password</password>
</jdbc>
Obviously replace XXX with your AS400's IP address and put in a username and password that has access to your AS400.
Fortunately no licence file is required.
I'm very pleased with the performance and it's reasonably straightforward to use. To populate a combobox for example requires some simple code like this:
<xp:comboBox
id="inputContainer"
>
<xp:selectItem itemLabel="- select -" itemValue=""></xp:selectItem>
<xp:selectItems id="selectItems7">
<xp:this.value><![CDATA[#{javascript:var con:java.sql.Connection;
try {
con = @JdbcGetConnection("as400");
var results:Array = @JdbcDbColumn(con, "DOMINO.CONTAINERS", "PRDNO");
return results
}finally{
con.close();
}}]]></xp:this.value>
</xp:selectItems>
</xp:comboBox>
Hopefully this will help someone one day :-)