So, looks like even after calling the (genuinely helpful!) VA platinum folks and them being able to give me the SQ booking reference, there is no way to select seats or pay for an exit row... I have a horrible feeling I will be stuck in a middle seat, and never, ever, booking a VA code share again - in fact it looks like a bunch of middle seats have been pre-selected for me.
VA. if anyone's listening, not a great way to get people booking through you!
Danny