Also keep in mind that service fees usually are not refunded and that several TAs and websites also charge for returned tickets.
If you do this play it right - so check in for your 'fake' flight, then find out that you need to go somewhere else, leave the lounge and check in for the original intended flight. I am not sure about Hong Kong, but otherwise at least in some european countries this could be seen as illegal behaviour if somebody can prove that you used the lounge without intention of flying on that airline/alliance. It is highly unlikely that anybody would check and find out, but it is still better not to leave a trail.
Considering all that it is likely not worth the hassle.