I have just tested it and it appears to work fine. I suspect your error may be due to the fact you are attempting to alter the default config, but aren't using the correct name - it should be org.jboss.seam.selectitems.config (n.b. org.jboss.seam.selectitems.selectItemsConfig will work as well). Alternatively use the schema version.
Cheers Pete. You were correct on both counts.