This is not a very useful answer, but might be relevant added info: You can browse services (that arise from your deployed processes) in the Admin UI and configure their "Run As" behavior. The Run As can be set to the Invoking User, a named user or System. I think you may be able to set the Run As for any long lived process to be the process invoker, which will in turn get passed through to any sub processes that get called. The downside of course is that you may not really want those processes running as the end user because it performs activity steps for which the end user is not authorized to invoke etc.
That said, even if you do propagate the invoker's identity through to the invocation of sub processes. Workspace might not track it. I think this is beacuse Workspace only tracks the process that it paricipated in (through inintiation of task assignment) not other processes that get a knock-on assignemnt. I need to check with the Workspace team to verify that suspicion though.