I've never had any PRC exit-entry administration official ever ask for the passport I used to get here, even if it is not patently obvious I arrived from Hong Kong (e.g. at PEK or PVG where HK flights land with non-HK flights).

In PVG you might even use the echannel where there is no opportunity for a passport inspection.

I think there's a note on Wiki's page on HRP that says new HRPs carry statement "This card is intended for its holder to travel to the mainland of China."

The air carrier on the US end will check your HRP (so it's best you have the new one with that statement) and the air carrier on the China end will check the ETA on your UK passport. I don't advertise the fact I have a foreign passport and a HRP, but I hardly doubt the air carrier on the China end is required to report it either.