Well, you can always use XML entity expansion and achieve what you want in a simple way.
Otherwise we'd have to introduce something "above" the process definition, be it a process definition group or a repository. I reckon a typical project has multiple process definitions, which will probably have common actions, swimlanes, etc. However, this is definitely not a simple addition.
On the other hand, entity expansion does exactly what you state in the second paragraph: the process definition document references an external swimlanes document entity. Upon deployment, the XML parser expands the swimlanes entity. To jBPM, this looks exactly like the swimlanes were in the process definition document.
That seems like the perfect solution, and I just wasn't aware of that XML parsing functionality. I'll test it out. Thanks Alex!