Wouldn't the blackish rump and dark grey back suggest against a 'pale-backed' taxon such as baicalensis? Also baicalensis should have a similar head pattern to leucopsis, with possibly a slightly more extensive bib - this wouldn't explain the solid dark on the ear coverts and moustachial shown by Mike's bird.
Although leucopsis can sometimes show darker smudges on the rear ear coverts (perhaps mostly first winter birds?), it is not normally as black as on this bird. My guess would be that this is a leucopsis x alboides intergrade. I don't think the black is extensive enough for pure alboides (which is not yet on the Hong Kong list).
For the record, I also saw baicalensis at Yuen Long last winter.
