You can deploy, alongside with your bpmn20.xml, a PNG or GIF file, containing the process definition image. This will prevent the users from looking at a generated image for the process-diagram. For the running processes, where highlighting of active activity is required, this won't work, off course. For this images, the BPMN-DI is still used for generating the diagram.
For the process-instance diagrams, you have 2 options:
- Remove the DI-information from the processes
- Implement the custom BPMNParser like you described