As with jBPM, the only component that depends on JBoss AS in BPEL is the service archive (.sar) that we provide for convenience.
If you take the libraries and configuration files out of that archive and put them in your app server classpath, you should be good to go.
Note that the BPEL requires a web services stack compliant with WSEE 1.1/SAAJ 1.2 and a JMS 1.1 implementation. Any full-blown J2EE 1.4 app server should do.
Conversely, a standard Tomcat distribution is not enough since it does not provide an implementation of the above specifications. You can always install one; be aware, however, that Axis is not compliant with WSEE 1.1. The EWS project (Enterprise Web Services) seems to be stalled.