Welcome to AFF, edt. The only way to secure an exit row, is to pay. You can chance it at the airport and ask for one, but if they're sold, you'll be out of luck.
The only way to upgrade to Business is to use points (which in this case you can't as you're on a redemption) or pay to upgrade (once again, out of luck there too). So you'll be stuck in Economy, unless they opup you (you don't mention your FF tier, but if you're not Platinum, the chances of this happening are slim).
Even though you're on a QF code, you're flying Jetstar metal (QF264 is JQ28). You won't have any lounge access either unfortunately I don't believe. There is a Royal Orchid lounge for HKT, next to gate 4, but it only opens 2 hours prior to Dragon Air departures (no idea of their schedules there). You can always try your luck if it's open.
This is the problem when flying JQ under a QF code, you don't get any of the QF benefits on the ground unfortunately.