Interesting dilemma. Not all that familiar with the Police SOP, but I think you have to be prepared for the worst. Granted, the local armed forces tend to "compromise" with local academic institutions more. But, as a policy, reservist deferments generally happen only for clashes-of-timing with examinations. (In other words, you can get NSmen who paid $20,000 to do part-time Masters but weren't excused for ICTs & ended up carrying textbooks during field training!)
Keep an open mind, & be prepared to do a lot of legwork cuz you're likely to deal with administrative folks (rather than anyone with executive powers) of both the reservist & polytechnic sides. Obtain a student-status letter &, more importantly, a relevant academic timetable which you can submit as evidence. Then write directly to your unit & apply for deferment ASAP. It's up to the SPF people to access whether they can release you. You may need to argue down the phone with a pencil-pusher, like this frustrated SCDF fellow who ultimately complained to the Cut Waste Panel:
http://app.mof.gov.sg/cutwaste/suggestionview.asp?id=326If they insist you attend, then you'll have to apply for official absence on the polytechnic side if I'm correct. Reservist is different from national service in that there's no rigid sense of "service exemption" associated with the former - well, except for the exit permit that shows you'll be overseas. That said, it's the unit commanders who actually have the final say.