I predict that AA's PEK schedule will similar to United's by October, so you'll probably have the option of a sane stopover in either direction. That having been said, it still makes better sense to go there on your way back.

Assuming it gets switched to a 5p departure, 2p flights out of HKG would make the cut.

Have you considered getting a PRC visa? You have lots of time, it's not extremely expensive ($150 plus agency fees), and you'll be able to start building a visa history. If you've never been to BJ before, you'll surely regret the fact that your stay is limited to 23:59... though 48 hours in SH is more than enough.