I have successfully deployed the .bar like described above.
It deployed all my custom classes needed to serialize/deserialize my BLOB values.
Somehow jbpm still throws "class not found" exception, although I am using jbpm 4.2 where the problem has been fixed (JBPM-2200 in JIRA).
Also the user guide suggests that this is the right way to make custom user classes available -> by adding .class files to .bar.
Has anyone experienced the same problem?