It is possible to use Jetstar when wanting to use 140,000 points to make a RTW using Oneworld.
SYD-HKT (coughet) is available using Qantas miles so there is no problem using your Qantas miles for this sector. I think the flight number is listed as QF253 on the awards.
Your problem is going to be finding 4 seats on the same flight.
140,000 x 4 = 560,000 plus 2,500 for assist.
AA/BA/QF and surprising AY(Finnair) were available when doing the online searches.
So my idea would be.
Download the oneworld timetable.
See what flights you would like plus any alternatives.
Go to the Qantas online awards and plug in the your flights plus dates, try for 4,3,2,1 passengers.
CX and RJ are the problems, as you can't tell and you would have to call reservations.
Flying around Europe you also have the low cost options of Ryanair and Easyjet; both are happy to take Australian credit cards.
Flying around Asia you have the option of Air Asia.
I have just used 140,000 miles to fly JQ/AY/BA/AA/QF.