Thank you for the quick answer.
I was also thinking about some sort of "custom persistence module", because each row/record in my database represents in-fact a running process and the fields of the record are simply the variables of this process instances. Each record update should push the process a step further - mostly using a decision point based on rules evaluating the old and new version of the record (exposed for rules as separate processs variables). I would like to use my record database instead of the original database schema used by jBPM persistence for storing the runtime state of the processes.
Do you see this as a feasible solution?