The ownership of Dragonair prior to the CX takeover was 38% CNAC and 28% CITIC, both of these companies were mainland owned and Hong Kong based.
China Eastern will be setting up a holding company that is no doubt based in Hong Kong which will meet the basic law requirements. HNA group has done exactly the same with Hong Kong Airlines.
The Basic Law was written to ensure that mainland chinese companies would not be disadvantaged.
A foreign firm without Hong Kong or mainland Chinese investment would have no chance, but Joyce has secured China Eastern. Their biggest shareholder is the Chinese government.