I'm in a similar situation but I'm looking for a different location. Some owners who post online directly might be willing to negotiate, it never hurt to ask. Some specifically said short term is okay but I'm not sure if they're within your budget, see for example:
http://hongkong.asiaxpat.com/propert...d=92666&type=1
or search for geoexpat classified,
http://www.expatflats.com/rental-property.htm http://hongkong.craigslist.org/apa/, or similar sites.
Another suggestion: pay an extra month's rent and use agencies--you can offer to pay for the owner's commission (the usual practice in HK is that owner and tenant each pay half month's rent as commission to the agent), i.e. you pay a whole month's rent as commission, that will likely give the owner some incentive to rent it to you for a six-month lease, and give the agent an easier job to find you an apt within your budget.
Good Luck!