I'm doing the same soon as well. I did some searching about this. The BA lounge isn't open for very long before the QR flight departs, but it's supposed to be dire anyway. I can't actually find if QR give you a voucher for the 3rd party lounge in their terminal but also doesn't seem like a great lounge. Seems the best is the AS lounge in the N gate area, but allow plenty of time to get to the QR gates since train transfers are necessary.

I actually regret it now, because I have to arrive in SEA pretty early and will have about a 5-6 hour layover as a result. That was enough time I could have just built in a connection to LAX and taken QF740 out of there, with either the AA FL or QF F lounge available.