In theory I think it would be doable but you need to remember that ProcessInstanceInfo is not the only table used by process runtime, we have WorkItemInfo, SessionInfo, etc. So doing it all would be rather significant change. And even if you do it then I think you won't be able to use process instance state to represent your business state as it is used internally and has some predefined values that must be kept. So you would still need to use process variables to store some data that will be used by the process instance (evaluate rules, etc)
What is the issue with having jbpm tables in your data model? Note that they can be in separate schema/db.
HTH