"..users have an option of selecting the Services they want to expose in jBPM as a Work Item"
This feature would be really nice to add to the jBPM Web Designer. Would you be willing to contribute? Your users then could for example select some services in designer and convert them to workitem definitions without having to leave their modelling environment.
We are currently working on visual service node creation in the web designer so this would be a nice extension to that.
If you want to contribute someting like this find me on IRC (#jbpm on codehaus - tsurdilo) and I'll help you get started.