Turns out, that the config parameters have been at the wrong position. The following standalone.xml works

        <subsystem xmlns="urn:jboss:domain:resource-adapters:6.1">
            <resource-adapters>
                <resource-adapter id="wmq.jakarta.jmsra.rar">
                    <archive>
                        wmq.jakarta.jmsra.rar
                    </archive>
                    <transaction-support>NoTransaction</transaction-support>
                    <config-property name="reconnectionRetryCount">
                        10
                    </config-property>
                    <config-property name="reconnectionRetryInterval">
                        900000
                    </config-property>
                    <connection-definitions>
                    ...