I'm not sure I understand your issue. The points component of an award booking will always be the lowest possible price - either the through fare if using airlines all on the one table, or the sector-by-sector fare is using airlines across different tables OR where mixed class is involved.
In many cases, where mixed classes are involved, it will usually be cheaper for the through fare to be applied.
If we take _flyer's example of ORD-SYD in J = 9232 miles = 126500 points (at today's table). The new itinerary is ORD-DFW-SYD = 9379 miles = 126500 points. If breaking it up would be ORD-DFW in economy for 801 miles = 12000 points + DFW-SYD at 8578 miles = 126500 = 138500 miles, so cheaper to do the through fare. Under the old points table I'd suspect the differences would be similar, so the through fare, even with one lg in coach, is cheaper than sector fares.
but that's not always the case. SYD-DFW in F = 189800 points. If QF re-routes you via LAX, so you're SYD-LAX in F connecting to LAX-DFW in economy, the sector fares would be 162800+18000 = 180800, so you'd be entitled to a refund of 9000 points.