Satay Sue - I just want to say "good luck" and (cue backwoods accent) "I feel your pain". We are American and moved from Australia two years ago.
Because we moved from Australia, and not straight from the US, we were unable to get priority at an American school such as HKIS. We also foolishly submitted applications at several schools and paid the application fees -- which are not refundable. Please confirm your kid's place on the waiting list before paying anything.
We were eventually able to get places at an ESF school and our kids (aged 5 and 8 at the time) - are doing fine now.
For practical advice, please go to your, or your spouse's, employer ASAP and let them know that the move is conditional on your finding a place for the kid in school. Most employers aren't monsters and will either i.) do something to help, or ii.) allow you an extra subsidy to work out a temporarily solution. Make HR and your boss aware of the situation and a corporate debenture may miraculously appear.
Good luck!