Goof point Donald. I would expect to have EL available. Looks like a great opportunity for contribution :)
So, I think a good place to start is to implement and interface of org.jbpm.process.builder.dialect.ProcessDialect and then add it to org.jbpm.process.builder.dialect.ProcessDialectRegistry. That would be the first steps, I hope...
Next is to make it visible for the processes and that would depend on where you would like to have EL support, for instance to have it available in Script Task you should add language support in SctiptTaskHandler inside handleNode method, similar to what is done for Java.
That would be initial work to add support of EL. It is not comprehensive guideline but just a starting point.