I understand how to use Cargo with Maven to deploy to a remote JBOSS server, however this assumes that I'm actively building.
In my specific case, I've already built the application, and have pushed it to a Nexus repository. Is there a way to configure Cargo or some other automation library to simply deploy using that artifact?
Yes. You can just download the deployable via http(s) from Nexus and then pass it on to cargo. Depending on what cargo component you are using this will be different (java, maven plugin, ant task...).
E.g. the Java APIenter link description here can be used directly in any Java code. or you could use the Ant task. The junit testing example has a useful snippet that you can use outside of junit as well.
And if you want to continue to use the Maven plugin you can do that too. The docs contains how to start as well as how to deploy to a container.
So you really have a wide choice of approaches available to you .. just choose what fits best to your existing tooling.
