Quick update on this for future reference - 4pm is indeed a very peak time at PEK passport control! The queues at the immigration booths were full, and everyone was being held back from those queues by additional barriers near the fingerprinting area. Not helped by my A350 parking at a bus gate near the D satellite, so adding an extra delay for the bus. I managed to go straight to the front of all the immigration queues, and got to my next flight with about 15 mins to spare, but if I had waited in all the queues, I'm sure I would have missed the connection.

And the comments above about the transfer area being on the left as you approach baggage reclaim are correct. Turn sharp left, go through a bag x-ray machine, then up the stairs to join the same domestic security queues as if you've just come from main T3 check-in.