You may find that those apparent fools at immigration aren't quite as dumb as you think. So unless your GF is Asian and comes from a country that has plenty of domestic helpers employed in Hong Kong your solution won't actually work.
Then of course there is the small problem of you and her "breaking up" and her running to immigration saying she's merely your DH and she felt she had no choice but to let you bang her.................
If you choose this route (root) to take please post on here again if it all gets messy, we need a laugh
