No special trick - I just searched it, it was available, and I booked it. This was back in late December 2012. Perhaps SQ have since realised just how good a deal this is, and pulled availability for it (given that "unavailable" literally means "award seats not offered on this flight", not just "no seats currently available")?
Actually, I think I know what is going on here, and the good news is that all hope is not lost for those wanting to fly the same route I did
When you search JFK-MEL on the SQ website, the only option it gives is SQ25 (JFK-SIN) connecting to SQ217 (SIN-MEL). As I detailed in
http://www.australianfrequentflyer....s-krisflyer/new-sq-777-300ers-have-47837.html, I noticed a while back that something funny was going on with SQ217 after it switches to a 77W on 01/07/2013.
While SQ PR ended up denying that SQ217 was getting new 77Ws with a Suites product (I forgot at the time to post this bit of information in that thread), and SQ appear to have corrected the erroneous "R" fare bucket I noted, it looks like they still have not opened any F Saver award availability for some reason (any search for F availability on just SQ217 after 1 July 2013 just shows "unavailable" at the Saver level).
So, I suspect that the "unavailability" of R/F Saver inventory on the SQ25-SQ217 flight combination is simply due to the unavailability on the SQ217 leg (not to mention after 1 July SQ217 is no longer an A380, so no Suites anyway!)
Searching just SIN-MEL after 1 July, R Savers do appear to be available on SQ227 (the only remaining SIN-MEL A380 flight after 1 July), and hence I suspect if you can find a SQ25 and SQ227 combo with R Saver availability then you'd be able to ticket it by ringing SQ. Of course, this means a
long connection (SQ25 arrives around 6am, and SQ227 departs around 9pm) - but at least it's (probably) still possible to fly the JFK-xFRA-xSIN-MEL route in Suites for ~117k points. Hopefully SQ will also re-open F Saver availability on SQ217 at some point too, so it will again become possible to fly the SQ25-SQ217 combo even though SQ217 will "just" be F.