Is it possible to get access to the process engine instanciated by the activiti-rest web application from my own web application? In other words, is it possible to share a process engine amongst different web apps?
That's exactly what the REST webapp is fo, having a centralized engine that is reachable through REST.
However, if you want the Java process engine, then will need to make sure that the process engine is reachable through the classpath on all webapps. Or you must start tinkering with jndi.