There are many mainland migrants in Hong Kong who speak Putonghua and who, theoretically, you can hire as maids, nannies or confinement ladies. But they are not considered "foreign helpers." They are local workers, who already have the right to live in HK. So none of the usual FDH rules apply -- you have to negotiate their pay and perks individually.
Generally, they are much more expensive than FDHs -- something like 10-12K per month, as opposed to 4K per month. There have also been concerns about the quality of their work, as they don't go through the whole training / health check / agency process that the FDHs do. Many are older Chinese ladies who are simply looking for some way to make some cash, sometimes in the short term.
I come from a large HK Chinese family. Despite the language barrier, ALL of my relatives have hired Filipinas or Indonesians to do the heavy lifting in terms of housework and childcare. The thousands of HKD they save is then spent on private Mandarin tutors -- so the Mainland Chinese are used in a more targeted way to teach language. Even Mainland Chinese families here opt for FDHs.
I don't think it's very easy to bring a Mainland helper to HK. There are a million reasons why -- too long to go into here. Mostly it's immigration control -- there are far more Mainlanders who want to come to HK than the city can absorb. HK's been burned by a few scams, like fake marriages or people overstaying tourist visas. HK worries that if they open the floodgates, they won't be able to control migration. Whereas Filipinas / Indonesians are kept on very tight immigration controls. But that's a whole other post....