Well I didn't know this was an option either until reading this thread earlier in the month.
I wanted to book QF64 JNB-SYD in J but they only had PE seats. This was for January next year and there were very little seats at all in January released, not surprising given it's a high holiday period. So I rang up wanting to book J from Cape Town to Melbourne which means three flights and with QF64 missing the J seat.
The QF rep requested a seat release which was immediately approved and booked the entire journey at 122k pts and $432.12 in fees. There was no assisted booking charge either since I had to call. I was also happy being charged in AUD instead of South African Rand (as would happen via online booking) since this will trigger the QF spend on the new Westpac Altitude Card I just received (for 2 QC passes which I'll give to family) and I'm now on my way to earning the 100k bonus sign up points for $5K spent which will basically cover this trips flights home