Currently I see no way to pass in data into the RESTWorkItemHandler for POST or other methods then using the URL of the REST service. Right now there's not way to invoke XML or JSON endpoints.
Added possibility to specify data that shall be send for POST and PUT methods. It shall be given as Content work item parameter. In addition to that ContentType should be given to set proper Content-type header while executing the HTTP method. ContentType work item parameter must be valid http content type e.g. application/xml. Commit https://github.com/droolsjbpm/jbpm/commit/452ef97e01924ab07f03363f296a211b9f039d4d
Verified on ER3.