This seems to be a bug, there's no reason why service tasks shouldn't work in embedded sub-process. It seems this might be the same issue as https://bugzilla.redhat.com/show_bug.cgi?id=1130061
This is already fixed in jBPM 6.2. I would recommend to contact support to see if they might be able to provide you with a fix.
As a workaround, rather than using the out-of-the-box BPMN2 service task, you might be able to use a domain-specific WS or Java Task for now.