Adding a depends element to jboss-web.xml seems to have no effect. Simplest way to confirm this is to add a jboss-web.xml with the following to deploy/ROOT.war/WEB-INF and start JBoss:

<!DOCTYPE jboss-web PUBLIC
"-//JBoss//DTD Web Application 5.0//EN"
"http://www.jboss.org/j2ee/dtd/jboss-web_5_0.dtd">

<jboss-web>
<depends>jboss:service=NonExistent</depends>
</jboss-web>

The ROOT.war deploys without issue even though the dependency is unresolved.