There have been some rather contentious posts on connection protection by oneworld on separate tickets on a couple of forums- the BA forum and on a thread started by me on the oneworld forum. The latter had a poster who was connecting from UL to CX and had written to both airlines, each of which confirmed that they would protect, even across different tickets. Imho, it's an unpublished oneworld policy- you might want to write to CX.

150minutes is more than enough! I've gone landslide to meet a friend over a meal on a similar transit- albeit it was on the same ticket. But you might want to call CX and cross reference the ticket with your BA ticket details, so that CX ground staff at HKIA 'know' that you are to transfer to BA. I'd bet that they'll protect you into a CX flight if you miss the connection. (CX because it would be their responsibility to rebook you- if AA's published policy on connection protection on separate tickets is anything to go by).