I've been through this many times - CX will send you to UO, whether its MPO, different offices, corporate sales, etc. They litterally cannot do anything with UO segments.

UO - cannot assign seat online. When calling (after an outrageous amount of time), sorry, please ask CX for a seat assignment.

The result is that you cannot assign a seat in advance on CX/UO codeshares.

Moral of the story - don't book CX tickets involving travel on UO. When I have no choice, I pay out of pocket for the UO segment and self connect - UO is pretty misserable if you're not in row 1.

And yes, no lounge access either way. The two general purpose lounges in DAD are nothing special, but they are priority pass, personally I like the one above Gate 4 a little better.

The Chase lounge at HKG is what I use if I have time when going out on UO - APM to 40-80, take the lift up from the APM to the main level, walk 20 m, up to the mezzanine and its right there. When you're done, same lift back down to the APM and one stop to the 200 gates.