BA also serves Beijing, but China is in general a problem. The best bet may be to have a PEK-HKG leg on another carrier (Dragonair or China Eastern) and proceed from there. Both KA and MU are members of AsiaMiles, so you could use miles from there to get a free flight if you wished.

You could also take the Beijing-Hong Kong train if you had time and wanted to see some of the country.