in standalone.xml, i've got a system property declared

<system-properties>
<property name="sql.user" value="sa"/>
... </system-properties>

it does not get resolved further down in datasource declaration,

...
<security>
<user-name>${sql.user}</user-name>
...
</security>

interestingly, all other properties resolve no problem, ie password, server etc.

there is a workaround (hardcode the username value!), thus setting the bug priority to minor.