Cheapest is to find Cathay Pacific flights using AA points to get to HK and then taking the ferry. Usually good last minute F/J availability but not sure about economy class.
EDIT: just did a quick search, lots of Y/J availability on the two dates you mentioned. You should be fine to book using AA. Transfer SPG to AA to top it up if you need to. I'd advise going J there and then using UA points to get back?
Last edited by roastpuff; Nov 3, 2014 at 12:55 am