Just an update on this - no love! The initial agent put the seat in but it dropped out as the charge had not been collected within 24 hours. Apparently, there is now no way to do exit row/paid seating when travelling on VA when booked through other airlines and on other airlines' ticket stock.

This is something that Qantas manages to be able to do (eg purchasing exit row when travelling on a QF flight issued by AA), so it is disappointing that VA systems aren't up to the same level!