They're still working on making sure everything is there - features were deprecated in ReportEngine before they were complete in REST.
I have not worked with the code you're looking at or with the Webi parameters stuff in REST, but here's a brief overview - the folks in the RESTful WS forum can help you with the details:
1. Log in to the RESTful Web Service using BO administrator credentials using the BI Platform RESTful service.
2. Get the details of the report from the Web Intelligence RESTful service. You can get them as either XML or JSON.
3. Get the parameters from the report - I'm not sure whether this will come in the response from step 2 or if you'll have to query for them separately.
4. Set the AutoPurge flag for each of the required parameters in the XML or JSON and sent the updated response back to the RESTful service.
5. Log out of the BI Platform RESTful service.
-Dell