Summary

So how did it all stack up? Here's the math I used to figure out what to do in the first place:

SIN-x-SDJ in Y, paid ticket: around S$1000, +7000 miles
SIN-x-SDJ in J, paid ticket: around S$3000, +10000 miles (150% on SAS)
SIN-x-SDJ in Y, SAS EB award: -30000 miles
SIN-x-SDJ in J, SAS EB award: -45000 miles

At 3.3 Sing cents/mile the Y award would have been a pretty bad deal, and paying triple for J wasn't really an option either. 6.7/c mile for J was twice as good, although simply paying for Y (and getting miles too) was a competitive option. But domestic Japan flight to SDJ were expensive, Shinkansen equally so, and the OZ option would allow me to (very quickly!) visit Korea and avoid NRT to boot. Two of the flights would be on the long side, I was tired of Y and this was supposed to be a vacation after all... so I checked with EB member service and they scored me a seat on SQ for the way back, shaving a day off my travel time, and I was sold.

After the trip, I then computed value added in mileage, ie. how much I would have paid individually for services rendered in comparison to flying Y and doing without.

SIN SATS lounge: +1000. Seems a fair price for a few sandwiches, a glass of wine and a copy of Newsweek.
SIN-ICN OZ J: +0. No redeeming value, I would have been better off in a decent Y seat. Very disappointing, esp. since this overnight segment was the primary reason I went for J.
ICN OZ J lounge: +5000. Especially the shower, but the Internet and other facilities were nice too.
ICN-SDJ OZ J: +2000. Bigger seat was OK, but most points just because I got off the plane fast and avoided the Immigration queues.
SDJ-ICN OZ J: +2000. Avoided check-in queue, the seat gets plus points mostly just because it was less cramped than Y would have been. And oh yes, the free cappucino and zundamochi too.
ICN SQ J lounge: +2000. A very nice lounge, but I didn't really have time (or need) for anything other than the net.
ICN-SIN SQ J: +10000. And you could easily have doubled that if only I had taken this as the night flight in the opposite direction.

Sum total adds up to 22000 miles, so adding on the 30k for the basic Y flight, that's 52000 miles of value for 45000 miles expended. So all in all, I still feel I got out ahead, although next time I'll be booking my J awards on an airline other than OZ... or (gasp) maybe I'll just pay for SQ Raffles outright.