The class org.jboss.seam.jbpm.SeamVariableResolver does not look up jbpm variables in a workflow and it is therefore not possible to use el in jpdl to get access to jbpm variables. The resolver should look up in the seam as well as the jbpm context.
We have developed a fix as explained in the forum post here http://www.jboss.com/index.html?module=bb&op=viewtopic&t=105448
I have attached the files to this issue with a comment per file. The deployment of the bootstrapper is done via components.xml like this