jBPM4 could use Blob type to store process variable, so if you want to do it, you can store a large data into process variable. But you should think about it whether it is necessary. Because you can't search that variable directly. Now jBPM just use Blob type to store the process definition xml files, process graphic and custom class files. Maybe some serilizable variable also store in the database, But they will be deleted when the process instance ended. The history variable feature is not yet implemente.
I think you should think about all of above to determine whether or not use process variable to store large data.